You can't blame Mark Waid for wanting to keep quiet about his Superman story while speaking with Revenge of the Fans. As far as we know, he's not in consideration to write a Superman movie, but he's better off playing it safe rather than revealing some or all of his idea and risk someone else stealing it. Whether the folks at DC and Warner Bros approach Waid for a Superman movie remains to be seen, but even if that opportunity never comes, at least he could conceivably still tell this story in a comic book. Granted, he hasn't written at DC in over a decade, but having also left a huge part on their publication history with his runs on JLA, Legion of Super-Heroes and Kingdom Come (to name a few), maybe he'll return to the DC universe someday.