When he said that, he was a Microsoft employee working on Windows being paid to speak at a Microsoft sponsored event. That's about as official as one can get outside of a board meeting. The statement was well publicized, so almost everyone knew about it. Microsoft had several years and numerous opportunities to correct the record on the matter, but they didn't, so the statement is assumed to be true.

They did a pretty poor job of managing the press around it though. The closest thing we ever got to a definitive statement was when they announced Windows 11.