Ha ha, there's a little misdirection in the title for ya. It's not the xenomorph aliens that are authentic (even though they are), it's the film "Aliens
" that the game is remaining authentic to. Gearbox has even gone so far as to get input from Ridley Scott, himself.
According to the trailer, Aliens: Colonial Marines
takes place 17 weeks after the events seen in the film Aliens
and special attention has been paid to how the environments look, how the guns operate, and how the xenomorphs behave.
I find it interesting that the film that arguably laid all the groundwork for modern first person shooters is now getting its own FPS interpretation that is desperately trying to remain faithful to the film. It's a cross-medium entertainment Möbius strip if there's ever been one.