Does it trigger a GA if used in this way?
Yes.
That just makes it an inferior version of other weak units with two movement points, like Jaguar Warriors or Impi.
Not quite. The Inca UU moves hills and Mountains like plains. So they can attack, win and retreat. With the exception of the mongolian UU and the spanish UU no other unit can do it. Well, for hills units with 3 movement points will work, but at mountains even they fail. So the the Inca UU has a rare strenght.