There is a third, rather unpleasant possibility here.
It’s possible that Trump always knew that there would not (or could not) be a complete, physical southern border wall. And he just said that to win votes.
Is that probable? No. But it is possible. The last politician I fully trusted was Ronald Reagan.
Well, obviously Trump knew you couldn’t build a wall over a mountain or river, but I think Trump meant that promise about building the wall fully, though.