No it is not an illegal move. Look closer at the castling rules. "The king cannot move through a square attacked by an enemy piece". In your position, the king did not move through a square attacked by an enemy piece.
Well, i know personally few players, ofc no one famous, who play chess for over 10 years and also dont know that O-O-O is in fact legal. When i tried to explain that both is possible, they didnt want to hear about it, so i let them live in their lie..
#3 he was not talking about that castling queenside is illegal. He was talking about that in THIS POSITION castling queenside is illegal (which is not true).
HI, in this game white castles long on move 12, it should be illegal right?