I don't think they will say there was another Enterprise before the movie one. Maybe April never actually captained any Enterprise.
Others ships appear in Star Trek: The Motion Picture among a series of illustrations depicting ships named Enterprise.
In comic he looks like an old spaceship captain. Perhaps he was the period of Captain Robau. Nobody knows if that period had some other ship name Enterprise.
I don't doubt that there is enough time gap that they could fit another Enterprise for April. It's a possibility. I just don't think they would go for that.
I think the movie Enterprise was intended to be the first one after Archer's.
I may of course be wrong. I'm just throwing another theory out there.