You have no clue what you are talking about.
The point is simple.
John Doe says "I understand the French Defense"
For that to be a true statement, he would need to be able to understand what White needs to do and what Black needs to do. He may, by choice, only choose to play it from the Black side, and maybe he plays the English Opening as White, but for John Doe to be making a true statement of "I understand the French Defense", he must at least be CAPABLE of playing it from either side of the Board, Black or White.
If after 12 moves of Poisoned Pawn French he recognizes the position from the Black perspective, and maybe knows a couple of good moves from there out of memory, but then turns the board around and sees the position from White's perspective and has no earthly clue what is going on or what White's ideas are, then he does not know the French Defense. He is able to parrot 12 moves from the White side. He still doesn't actually understand what he is doing!
There's a major difference between the two. I understand the Stonewall Dutch. I understand the Najdorf Sicilian. I understand the Classical King's Indian. I can only parrot the Leningrad Dutch!
I am sorry but I disagree with some of what you said.
"Bobby Fischer" played 1.e4 almost exclusively. He did play 1.c4 before and I believe on very very few instances he played 1.d4.
He understood the Kings Indian Defence as black very well. Does that mean he had to play it from white's point of view? No it does not.
As a chess player, you can figure out from the black side what white is planning to do with his moves.
Which is why they tell beginners to pay attention to their opponents moves.
I do agree with your idea. Your idea of learning the position from both sides can help in knowing your line better. Yet It does not mean it is forced/impossible to learn it only from 1 side.
Which goes back to the notion that learning from 1 side is possible and can be done.
Some players in chess play unsound openings. I frankly do not wish to learn their unsound openings as white to better understand it as black? An I do not have to.
However, In saying all of this I believe this disagreement we have is totally irrelevent to the thread.
The question is about whites opening advantage.
Which I believe white can have. Since white has the advantage of the first move.

But then, no-one is perfect and to assume that they are is silly. Hero-worship of GMs is just as childish as hero-worship of pop stars. They are human and they make logical errors.
I think that Xplayer sometimes over-reacts to criticism and adopts a sort of hostile, opposed view. This is what happened to me when we had our differences about a bit of analysis. There's no reason to get upset and if a person does, he or she has to accept that someone else might very well get upset at them. But people are coming from different angles and different ability levels. We should probably try to get along and make this a forum from which everybody can learn, which means accepting that people may have different angles and different ability levels but that doesn't necessarily make them wrong from that angle and maybe within that ability level. Even when I just got confused about what side of a position someone was supposed to be playing, that was my stupidity but it isn't a very big deal.