Sounds like the theory supported by Richard C. Hoagland.
Not saying that he is wrong (it is, after all, a theory) but well, still, whenever he's on C2C and talks about Mars, one of the theories he constantly brings out and of course, supports is the exploded Planet theory in which Mars was a moon of.
It was Van Flandern, RCH's one time partner in space speculation. But the idea was around long before Art Bell made his first radio.
There is not enough mass in the asteroid belt. Jupiter played a large part in the prevention of the asteroids from forming even a small moon like object.
No, it is not a theory, it is, at best, a hypothesis, and at worst pure speculation.
Scientists use the word "theory" in a very specific way that is often confused by laymen.