In the episode, he's shown a propensity of asking questions he knows the answer to. Sometimes it's charming, sometimes its infuriating (like insisting Angela tells him to leave before he leaves).
Honestly, as I typed this, I realized the same thing could be said about the comic. Why didn't Dr. Manhattan know Ozy's plot after Ozymandias explained it in the end? Or after he went to Mars and was clear of the tachyon interference? There's tachyon equipment right outside of the Abar's house, so that could explain his confusion/inability to know there was a dude behind the cannon.
I really liked Prometheus
I'd think if any poster could appreciate a writer/showrunner who frustrates the hell out of his audience, it would be you