Mark Millar on Superman Movie Sequel

Mark Millar just posted that he got turned down for writing the Superman movie sequel because he was too associated with Marvel:

Well, sadly, I'm a Marvel guy and we were surprised to find out that WB couldn't hire me for a DC property. They were incredibly nice and superbly apologetic about it, but when they discussed the matter seriously DC explained just how associated I am with Marvel Comics at the moment and it's against company policy to hire the competition. It's absolutely nothing personal. I spoke to some friends at DC and they explained this has happened with a couple of big Marvel writers in the last couple of years and I absolutely respect that.

My question is: How is that different when super-DC guy Geoff Johns wrote and produced for the Blade TV series (a Marvel property)?