I'm not disagreeing with that.
I'm saying that - at some point on its way to the receiver's hands - it's literally nearly impossible for the ball not to cross the plane.
EDIT: Plenz's example (below) is the only way the ball doesn't cross the plane inside the pylons.
Josh is on the boundary, slings one sidearm to Diggs in the corner of the endzone, ball travels in the air entirely out of bounds. Diggs toe drags, catches the ball while it is outside the boundary, the ball traveled the whole way outside of the boundary...that's a TD as long as he completes the process of the catch.