I have to agree that KS wouldn't say anything negative in this case. If Abrams lets him attend a very advance screening of STXI (the movie is supposedly still missing most of VFX shots), then they must be good friends and it's unlikely that Smith would bad mouth the movie.
But on the other hand he didn't have to say anything. And if he didn't like it I think he would have kept quiet, especially since the movie is still incomplete. And he certainly wouldn't use words like "awesome", "phenomenal" and "fantastic" for a movie he didn't even like (he actually used all 3 of those words).
And his "Anybody who was worried doesn’t need to be worried about this film
" comment was directed at us Trekkies.