That's a little inconsistent with what I understand about alternate timelines. As I understand it, having shunted himself into the past of an alternate timeline, Ben would then remain in that timeline and 'ride' it back to the present, where he would be confronted with a future where he'd only briefly been the Thing, and where, presumably, the FF had therefore either broken up or gotten a replacement member. In order to return to his own time, he would have to go SIDEWAYS in time, back across the timelines to the one where he originated.