but predators cannot close fists because of claws
anyway, if i have to be serious - look how he holds the spear in the same scene:
so "realistic" for a predator means "unrealistic" for a human.
(this reminds me of a recent 1/4 T2 statue with a minigun, where people said that "realistic" body position is better than an intentionally inconvenient for a human, but actually used in the movie)
not only thumbs but all fingers. i slightly doubt it was intentionally a KPH hallmark because it seemed to be impossible (or rather inconvenient) to do any other way not only because of the claws, but also because KPH fingertips might end earlier than the suit glove's fingertips. like on original alien hands.
i thought X-legs (walking with knees looking inside) was a KPH mark as well, but now i also think that the suit probably didn't allow it to look another way as well.
update to both parts: looks like the thumb could be bent, so "thumbs out" definitely was a special feature by KPH, like
ridetofight said. though the only examples i found were outside the movie, but they definitely show the thumbs can be bent.
(thanks to
Moguchiy_H for the last photo, i've never seen it before or later)
(it also shows the X-legs i mentioned, they are best visible during his walk with King Willie's head in the movie)
sorry i'm shutting up and taking my antiblabbingaboutpredators pill