Right, let's assume that Biff did return to the alternate 2015. He obviously made a point to return to the same spot where he left, and since Marty, Doc and Jennifer were where they were, it was easy for them to just jump back in the time machine and go back to 1985. Even if it was the alternate 2015, their time in it would have been mere minutes, and nothing in the area where they were changed enough for them to notice, especially since their focus at that point was not their surroundings, but getting young Jennifer out of Old Marty and Old Jennifer's house and back down the street to the time machine.