Is there a rule that says you’re not offside if you can’t get out of the way? No. Every player is responsible for where they are on the pitch

Actually, there is:

interfering with an opponent by:

preventing an opponent from playing or being able to play the ball by clearly obstructing the opponent’s line of vision or

challenging an opponent for the ball or

clearly attempting to play a ball which is close to him when this action impacts on an opponent or

making an obvious action which clearly impacts on the ability of an opponent to play the ball

If the offside player doesn't obstruct an opponent's vision, play the ball, challenge an opponent, or make an "obvious action" then there is no offence.