don't study openings until level 2000?

The opening you play wont matter if their 2400+ standard over the board.

    You loose anyways

Blitz is not as same as classic time control where players spent half of their time on first 10-15 moves.

"... A remark like 'games are rarely decided in the opening' does not really do justice to the issue. ... even if an initial opening advantage gets spoiled by subsequent mistakes, this doesn't render it meaningless. In the long run, having the advantage out of the opening will bring you better results. Maybe this warning against the study of openings especially focuses on 'merely learning moves'. But almost all opening books and DVD's give ample attention to general plans and developing schemes, typical tactics, whole games, and so on. ..." - IM Willy Hendriks (2012)

The question is ,,,,,,, Is rated 2400+ standard ....????

    Was that a standard game?

   Make a tremendous difference when that IM has time to think.

Where are those puzzles from?

Books are written because they sell, not because they're appropriate.

But that strange logic aside, certainly anyone who claims to know nothing about openings until 2000 is lying. You need to study all aspects to improve, and that includes openings.

Is selling perhaps easier for books that are not denounced as inappropriate?

 I'm guessing there are more books on the opening than anything else. It's what buyers (and beginners) want, and they're relatively easy to write.

So a few players who have made that mistake warn against it... practically no one listens (apparently). You see it all the time, a beginner asking for the one new opening that will completely change his results. Or some noob on a Carlsen youtube video commenting how Carlsen is the best because he has the best openings.

Beginners instinctively believe that openings are the first thing you should study, and are also the most important.

defintely, a player who indeed does not memorize lines and does not play by book,  at least knows very well different kind of position it can get from different openings and pawn structures including ideas, and that helps him a lot to shape correctly his own play according to his own style or at least to not be surprised by some trap.

You're right, some opening study is worthwhile far under 2000.  But don't use the fact there are books on openings written by masters.  The USCF and other organizations have long stuck it to lower rated players for the financial advantage of pro players writing books.  There have been books over the past half century with titles similar to Open your game like Bobby Fischer, Play Kasparov's Openings, which is potentially awful advice because their variation choices may no be suited to lower-rated players.

But you should know a few openings so that you can become comfortable with them as you play them over and over and get a little edge on your opponents.

Personally, I try to keep my opening choices similar to each other. As Black, against 1 e4 I like to play the Caro-Kann (1...c6 2...d5) and against 1 d4 I like the Slav (the same 1...c6 2...d5).  As White against both the Caro-Kann or the French I like the advance variations where White plays e5.  As White against 1 e4 e5 I like the Vienna or Bishop's Openings, both of which attempt to get in an early f4.  Against the Sicilian I like the c3 (Alapin) Sicilian and have been exploring the Closed Sicilian - both of which avoid the more usual variations that some players memorize in depth.

I generally do NOT memorize variations in great detail, I memorize the ideas behind the openings. For example, as Black in the Caro-Kann a Knight can often find a good outpost on d5 while denying White outposts on e5 and c5. White's pawn on d4 is often a good target, and playing your c6-Pawn to c5, attacking White's Pawn Chain at its base, is the way to open up the game and Black can generate a Q-side attack.  I don't need to know a precise sequence of moves to do those things.

Just to give you an interesting viewpoint on this:

In my chess club there are two players, one rated just under FIDE 2100, the other just under FIDE 2200. 

Both are not strong in openings. They have no special "pet lines" and know very little. 

How do I know this?

I play them regularly and regularly get excellent positions against them from the opening. 

Sometimes this leads to me defeating them quickly!

But they are higher rated, and beat me more than I beat them. 

What generally happens is that I do get a fantastic position with all of the strategical perks that I'm looking for - a great center, well positioned pieces, classically "better" pieces - 

And it is at that point that I start running into their strengths - 

The resilient defense, the staying power in a slightly inferior position, the resourcefulness, the conversion into a more holdable endgame. The speed in endgame play (we play blitz generally), the understanding as of the correct simplification, the spotting of counter chances. 

They can turn a game around, to my great frustration!

And they keep saying - "I know that below master strength, the kind of opening play I have is quite good enough". Sure - they simply know how to play chess once the opening is over (in case they didn't get completely destroyed, of course). 

I do manage to beat them also later in the game, on occasion. They are not invincible. But they rated correctly, no doubt about it.

I prefer opening manuals like MCO, FCO, NCO, etc. If one player deviates, middle game starts for me. If one knows basic tactics, basic endgames, opening study makes chess more interesting. Who wants to play 1. e4 e5 or 1. d4 d5 again and again.

On another note - I have here a thread of defeating an IM from a "non-opening", where I tried to use an idea from one opening in quite another, not knowing the difference. It was the tactical resourcefulness that later prevailed, in giving me a really nice win. 

So you're ment to get to expert level without being an expert?  Sounds difficult.

I think openings are very important almost from day 1, at least become familiar with and learn to defend against the common openings. Checkmate patterns are very important as well. 

And I assume any GM could beat me with things like 1.f3 2.Kf2

But as far as improvement, it's sensible to not leave any large holes. Try to study a little of everything.

Humans kinda already have a knack for playing openings , knowing basic principles is good for us all..

    Thorughly studying openings and all their winning lines would take a lifetime .

Do you have experience with the writing of various sorts of chess books?

Could it be that these few players are not correctly describing a mistake that is generally perceived by authorities?