I can't tell anymore if we're on the same page, to be honest. There are two sets of parentheses, one inside of the other (Gattaca forgot to copy the last one, but you'll see if you follow the link below). The entire exception, if you cleaned up the double-parentheses, would read:
Exception: When a ball carrier is airborne as they cross the sideline (including a striding runner), forward progress is determined by the position of the ball as it crosses the sideline.
If the runner is airborne (again, it's unclear what "striding" means, in this case) as they cross out-of-bounds, then the ball is placed at the spot that it crossed the sideline.
http://www.myiafoa.org/rules/2018/ifaf2018/4.html