[[WMG: Robert (Revenge of the Skrulls) is a Shi'ar or other form of benevolent alien]]
He knows way to much about the Fantastic Four and even the Kree Universal Weapon, sure it could be Rule of Funny but even Reed calls him on it. And while it is possible he also reads anything Mr. Fantastic posts on scientific journals (which might evn include said weapon) it seems like it would be far easier then he is just a alien not a fan who knows things even Reed is surprised about. Granted he is a kid and Reed might not peg his scientific journals as something on "the internet" but that's a odd stretch.
[[WMG: Johnny actually did recognize Ronan during the confrontation in the Super-Skrull episode.]]
Johnny is an incorrigible prankster whose favorite sport is driving people crazy. He knew Ronan was too wrapped up in his vendetta do do anything before he got his enemy to recognize him, and so he has some fun with the Kree. For instance, he "almost" recognizes him various times.