A difference between a 1700 and 2200 blitz player

Sure, this was a chaotic game.
In normal games though, I'd say the stronger player is just more patient. They trade less, they build their position more... and it's because they're familiar with more ideas, so they can notice these things without trying to do anything immediate.

I think many players have enough experience but still their ratings doesn’t increase and I think at least for blitz, this is very much related to their opening repertoire and preparations for various lines, not about quality of tactical/positional understanding in general (unfamiliar situations).

Give me a random repertoire, even make me play openings I actively dislike / am uncomfortable in, and as long as it's not stupid (put a pawn in the center fairly early, develop, castle) I think my blitz rating would only drop ~100 points. Definitely less than 200.
So instead of worrying about deep preparation, they could just avoid chaotic (and frankly stupid) openings like 2...g5.

Didn't Kasparov just lose a blitz game in about 7 moves?
He scored 0.5/9... I feel like even I could do that on a good day!
But what happened to Kasparov is a cautionary tale: don't play a blitz tourney while on tilt lol
or...more simply
Don't play blitz


Or even at start, I had a miserable attempt to trick him by playing qd6, but my opponent fell for it (he got greedy). The reason he missed was that he missed the check. The reason he missed the check was that in a normal game after 5-6 moves if king is in center, it is well protected from such vertical checks. So it was really unfamiliarity of the board that make it difficult for him to see such a cheapo (he had plenty of time then).

Kasparov was good when there was no engines and people couldn’t prepare with it. That’s actually a very good point, if you are not excellent at opening, nowadays you will lose. Kasparov tried to work normal opening out but he failed. If he was playing stupid openings maybe he was more successful.

Give me a random repertoire, even make me play openings I actively dislike / am uncomfortable in, and as long as it's not stupid (put a pawn in the center fairly early, develop, castle) I think my blitz rating would only drop ~100 points. Definitely less than 200.
So instead of worrying about deep preparation, they could just avoid chaotic (and frankly stupid) openings like 2...g5.

You both had time to think before move 8, and both chose not to use it. Perhaps losing on little time, to begin with, is important to you both.
Learning how to win on time or from misclicks/mouseslips/blunders won't help you overall. It might increase your blitz/bullet rating, but that's not the same thing.
I get the message you're sending, just don't believe the fast time controls help you "improve" unless you're testing your ability to think quickly and/or use your time wisely.
However if they are out of preparation then they miss many simple tactics. For example look at this game, I had mouse slip at the second move and played g5, then my opponent slowly convert a completely winning position to a completely losing position and he was 2200+, like me.
[Site "Chess.com iPhone"]
[Date "07/11/2021 10:28AM"]
[FEN rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1]
[White "PacerSimo"]
[Black "TCSPlayer"]
[Result "Black Wins"]
[WhiteElo "2219"]
[BlackElo "2245"]
[Termination "Black Wins"]
1.e4 {2:58} c5 {2:59} 2.Nf3 {2:57} g5 {2:58} 3.Nxg5 {2:54} d5 {2:57} 4.Qh5 {2:48} Nh6 {2:52} 5.e5 {2:46} Nc6 {2:50} 6.e6 {2:45} Qd6 {2:42} 7.Nxf7 {2:36} Qxe6+ {2:38} 8.Be2 {2:28} Qxf7 {2:35} 9.Qh4 {2:23} Nd4 {2:30} 10.d3 {2:12} Nxc2+ {2:29} 11.Kd2 {2:10} Nf5 {2:17} 12.Qa4+ {2:08} Bd7 {2:15} 13.Qxc2 {2:07} Nd4 {2:14} 14.Qxc5 {1:57} Qxf2 {2:10} 15.Qxd4 {1:23} Qxd4 {2:08} {Black Wins}
(I cannot put it in a good format on mobile app, if someone can do it in comments I’ll be thankful).
The point was that, the chaotic position arrived after a few moves, was not a normal position we see every day, so at that point we both were like 1200 players, I mean like without any opening knowledge, also tactical patterns weren’t the familiar patterns arriving after opening. We both missed some tactical chances (I believe). If it was a long game and my opponent had more time to think, then he wouldn’t miss that many chances, but this shows even at our level, we are not performing much better than noobs if opening knowledge is taken away.
Never think that you have a less chess understanding compared to a much higher rated players. Instead find your own weaknesses and improve them. Also don’t give up easily, this is a key to success. For me it even happened to win a game against a 2300+ player even after blundering a queen for a bishop, that is usually an automatic resign (since unlike 1500- level, such blunders are very unlikely in 2200+ and the opponent easily wins).