I'm calling BS on this one. None of the rumors have had in the past (from sources proven correct about their PS4 specs) have suggested MS having GDDR5 to begin with. It was DDR3 and ESRAM.
There is 1 of 2 possibilities here. This guy is full of shit and just wanted some attention. Or this guy is an insider, MS switched over to GDDR5 to match the PS4, and he is just saying this to make it look like this was the plan all along. I'm thinking the first, but the latter could explain why MS delayed their reveal. Of course, that was also explained by an insider saying MS was behind schedule on game development for the NeXbox.