Given that it was 24 years after the fact, he may have been misremembering. Surely if Roddenberry had come up with Sulu's given name, it would've been mentioned in the series bible or The Making of Star Trek
, or would've been mentioned by Roddenberry himself at some convention or other and been known about by fandom. But before McIntyre's The Entropy Effect
came along, the leading theory in fandom was that Sulu's first name was Walter.
The only way this claim could be true is if Roddenberry had decided to keep Sulu's first name top-secret for some reason, yet had taken Vonda McIntyre alone into his confidence. Which makes no sense whatsoever. So I think Takei just got different parts of his past mixed up, which is something human beings do all the time.