Just cause people (in the movie) theorized that decapitating deadpool wolverine and sabertooth would kill them, doesnt mean its true... they were just taking a shot in the dark on how to kill them. either way look and the post credit endings (multiple endings depending on which print you watched) whether it was the wolverine one in the bar with the cheesy line "No, drinking to remember" or one with deadpools body inching towards his head, his eyes opening and uttering "shh" so obviously he is still alive