What makes it more than just a flashback or recap is that Immonen told them by way of Lex Luthor telling fairy tales to his infant daughter Lena, and illustrated the origins in one- or two-page comics fashioned after Winsor McCay's Little Nemo in Slumberland.
Brainiac ("Your brainwaves bother me some!"):
Doomsday (HE'S ADORABLE):
The Cyborg Superman ("um!"):
Toyman (I love Superman assaulted by fireworks):
And lastly, Lex Luthor:
One of the things I love about this is that, whereas the civilian Metropolitans - like the police, or Lois and Clark - are dressed in Nemo-esque fashions, Superman and his villains are largely wearing their actual outfits from the mainstream comics. Including Metallo, who actually went shirtless and wore a purple vest during his first post-Crisis battle with Superman, a look which only actually seems to fit the character and the environment if it's 1902 in a Winsor McCay cartoon.
Honestly, these are just fantastic. I wish I had copies of the (I think) J.H.Williams' Nemo pastiche from Promethea, or the one from Gaiman's Sandman (which I think was Dringenberg and Jones III, wasn't it? Anyone?)