It's a decent twist, and eventually we do find out why the attackers so closely resemble humans. The film's script, which was penned by The Adjustment Bureau writer George Nolfi, is packed with enough genuine scientific jargon to send you down a post-viewing Wikipedia K-hole. Phrases like "scanning people on a molecular level" and "cloaking technology" get thrown around. There's a veneer of authenticity that elevates the mumbo-jumbo above the type of dialogue you hear in a Syfy original.
But this is still an action movie. When our geek hero and his new army friends get backed into a corner, what do they do? Build cooler weapons. Because even if the enemies aren't ghosts, the solution in movies like this is usually pretty simple: get a bigger gun.