Well I had a look at his twitter and didn't see anything about 500 gflops. He did say he doesn't know anything about GPU power though.
Also I would probably think most people are going by the NeoGAF breakdown of the Wii U, where it says there would be a maximum of 352 gflops, going by the best possible specs it could have based on what they have uncovered.
http://www.neogaf.com/forum/showthread.php?t=511628
Here is the CPU as well
http://www.neogaf.com/forum/showthread.php?t=513471
PSN: Osc89
NNID: Oscar89