That is how the rule is defined, and usually I agree with it.
What makes this situation different in my mind -- the WR was not bobbling it, nor did the ground cause the ball to come loose. The WR caught it, got 2 feet in, stepped out, then the defender (who was also out of bounds) finally swiped the ball loose.
The "complete the catch" process makes sense, but this is not what that was. To me, this is closer to a player catching it, then a defender following him out of bounds and punching it loose, and it being ruled incomplete.
Either way I don't care enough to go full lawyer about it. And anything that upsets Beamer pleases me.