I think it depends on whether the player actually makes a legal move at the same time. If the player attempts to make a legal move, but knocks over the piece, and then presses the clock without making the move properly, then that is considered as an illegal move. But if the player makes a legal move, but accidentally knocks over or displaces some other pieces at the same time, then I don't think that is the same.
I agree.7.7.1 If a player uses two hands to make a single move (in case of castling, capturing or promotion), it shall be considered as an illegal move.
If he move a piece with his right hand, and press the clock with his left hand, is this an illegal move?
I guess not, since "6.2.3 A player must press his clock with the same hand with which he made his move." is a rule concerning the chess clock, not the move.