What reason would Urban have to pull Mitchell out of his ass?
To screw with people.
Also, from what I recall reading, Urban is a fan of the show from way back. I don't know how much of a Trekker Simon Pegg is, but he's also a long-time science fiction fan (actually, "proud flag-waving geek" would be a better description
). So it wouldn't surprise me if either of them were prone to joking around in interviews, pretending to give away information and then pulling back to tease us.
In fact, it wouldn't shock me at all if either of them were reading all the speculation online and laughing at how we're tying ourselves up in knots trying to figure out the identity of the guest characters and the plot of the movie.