Personally I think, as described, the play should be ruled a TB anyway as the defender did not complete all the elements of a catch until he fell into the end zone and maintained control of the ball.
When-and-where a catch
happens is not always the same as when-and-where we finally
determine that a catch has happened.
In most action scenarios, we know this already. Suppose a player jumps for a ball in the end zone, grabs it, touches one foot inbounds, brings the foot back into the air, and crashes to the ground out of bounds next to the photographer. The field judge observes that he keeps control of the ball as he lands next to a zoom lens. That is where the player Completes The Process. That is the point in time at which the field judge
rules that a catch has occurred. However, the catch did not occur there, out of bounds. It occurred
earlier, and inbounds, because that's where the player's foot touched down. We don't wave off the touchdown just because he Completed The Process out of bounds.
Likewise, just because the intercepting player in our momentum scenario doesn't Complete The Process until the end zone, that doesn't mean the
spot of the catch was there. It's back in the field of play, where the ball was when his first foot touched down.
EDIT: I know the manual says to make it a touchback when it's close. Just keep in mind it's not always close. The same kind of catch could happen at, say, the B-4.