He's Mephisto's puppet, so of course he knows! Didn't you see his smile in the first issue, and his sinister voice?
There's no way you can kill Galactus that easy. That is a clearly a false memory. And if that's not the case, then it's probable that Galactus wasn't even the real one, but another "lie" created by Mephisto to be used like the Squadron Supreme itself.
This couldn't have been caused by Echo, it's obvious Mephisto did it, somehow.
By the way, I have another suspiction for the events to come. It's obvious the Avengers will restore the original world, but I just realized a way the Squadron Supreme MIGHT escape Coulson's and Mephisto's control. In issue #3, Silver Witch said Blur was unnatural and he shouldn't even be alive. Well, how about if by mixing the "Flames of Rebirth" from the Phoenix Force and the power of the Starbrand, the Squadron Supreme are turned into real people and neither Mephisto or Coulson can control them anymore.
It would be appropiated if the Pnoenix's power burns out these fakers and then, makes them reborn as "real". Don't you think?