dunno
Why is this move wrong?

Do you mean, in the video the person that said it was a "huge" mistake did not tell you why, or you are seeing if we can guess what they said was the reason why it was a mistake? I think that in your example of possible continuations, you are overlooking other moves by the players. For instance, I would not move my queen back, but to c4. There are probably a lot of other alternate moves for White, but I can not see why that particular move by Black (8 ... e5) is a Huge Mistake. There are other moves that are much worse, if you ask me.

Play it out one more move...11. Bxf6 Bxf6 12. Qxd6; white is up a pawn
or:
11. Bxf6 gxf6; kingside is now a huge target with ideas such as Nh4 - f5, Nd5, Qf3 and possibly some pawn storming. Black is definitely worse off here (IMO) so at best black is playing a pawn down.
However with 10...Ng4 I'd say black is ok, the problem with the move mainly positional...creates a long-term weakness on d6 and the thematic d5 push is really no longer possible.

Do you mean, in the video the person that said it was a "huge" mistake did not tell you why, or you are seeing if we can guess what they said was the reason why it was a mistake? I think that in your example of possible continuations, you are overlooking other moves by the players. For instance, I would not move my queen back, but to c4. There are probably a lot of other alternate moves for White, but I can not see why that particular move by Black (8 ... e5) is a Huge Mistake. There are other moves that are much worse, if you ask me.
No, he didn't explain, it was supposed to be in another video, but I could'nt find it.
Thanks, bondiggity, yes a pawn up is pretty good, but not the huge mistake I was trying to find. But i guess that's what he meant.

Yeah I was going to also suggest in the final position that Bxf6 is a good move to ruin black's pawn structure.
I guess black might recapture with the bishop and then will have some compensation for the d pawn in the two bishops...
Because keeping material even kills his kside pawn structure, and the white squared weaknesses will be great for white's knights. So, comparatively speaking, I think it's better to keep white's knights out for the price of a pawn.
But - imagine that black played e6 instead of e6 - in this case he would recapture with gxf6 probably, because the e6 pawn covers f5 and Nh4 doesn't have Nf5 as a threat.

I would tend to agree that it's a huge error, but it's more of a huge strategic error which won't be punished in the next 5 moves or so.

Like you said, he said "mistake", not "blunder". Black is not OK after the simple 11. Bxf6! and Black, unless he is a gambiteer (or an idiot, whichever comes first) will be obligated to play 11... gxf6, permanantely destroying Black's pawn structure, weakening the King, making the Black Bisop bad, and removing a key defender of d5.
I was watching a video on the Polgar variation of the Sicilian & the person doing the analysis said that 8... e5 for black was a huge mistake. Why.