Did the Detroit Lions Really Win Sunday?

My understanding of the NFL rule that was applied Sunday to take away the apparent touchdown by Detroit's Calvin Johnson is the following.

If a player goes to the ground in the act of catching a pass (with or without contact by an opponent), he must maintain control of the ball after he touches the ground, whether in the field of play or the end zone. If he loses control of the ball, and the ball touches the ground before he regains control, the pass is incomplete. If he regains control prior to the ball touching the ground, the pass is complete.
(Reference: http://espn.go.com/blog/nfcnorth/post/_/id/16142/like-it-or-not-megatron-call-was-right)

The defense of the call is based on the second half of the first sentence. My objection to the call is the "if statement" in the first half of the sentence - the premise for the balance of the rule - is not applicable. Mr. Johnson was not in the act of catching a pass when he went to the ground, he had already caught the pass. The rule cited, then, does not apply. Rather than being an incompletion, it was a touchdown. If what I view as the correct interpretation had been applied, the game would have changed significantly, but I do not know if it would have resulted in a Lions victory. No one can know that even though the odds are highly stacked to support such a conclusion. Football is a game of improbabilities. Had the Lions been given the touchdown, improbable as it would seem, the Bears - with a superb placekicker - could still have won. We will never know.