I honestly can't tell much of anything about actual gameplay from what looks like a bunch of cutscenes. I suspect, though I can't be sure, that this is based on the Fireteam engine. (Notably, Aliens: Fireteam, or whatever it's called, Left 4team Dead

is getting a proper campaign soon if not already.)
My "4th generation Genestealers" quip concerns the humanesque Xenomorphs able to wield weapons. Which in Warhammer40K games (Space Hulk or otherwise) starts in the 3rd generation of the "Genestealer" aliens (from the Tyranid faction) that Games Workshop invented to sub in for the Xenomorph aliens.
(Come to think of it, those in the trailer might be more like 3rd generation as they're clearly still alien-looking. 4th Gen Genestealers can easily pass for human if they're careful, being more like vampire coven leaders.)