Do you not understand the rules? Once a defender attempts to play the ball, it doesn't matter how "offside" the striker is, the attacker automatically becomes onside by default. This rule has been in place for donkey years.
It hasn't. The striker is offside when the initial ball is played. The deflection doesn't matter as it's unintended.
It's like a through ball coming off the leg of a defender who's got an interception wrong, rolling through to a striker five yards offside - it's still offside as the forward ball was initially made by the opposition player.
It comes down to whether it's a 'deliberate play' on the ball.
All that said, if Mount doesn't touch the ball as some have said (looks it from that angle but haven't seen others), then the point is moot. If the defender just hooks the ball over his own head, even if under pressure, then Giroud is onside. If it's deflected off his toe after Mount gets the first touch, he's offside.