Okay, so we have a rumor that Tim Cook was visiting Valve, and a guy at Valve saying he asked a bunch of people and nobody said he was meeting with anybody at Valve. So it's possible he was there, but just didn't meet with anyone. Seriously, if you're Tim Cook, you can probably just wander into any building you like, and people assume you belong there and don't try to stop you. He's Tim Freaking Cook. Nobody's going to ask him what he's doing there, or why he's looking over your shoulder as you work on your super secret Valve console. That would just make you look stupid since clearly he's supposed to be there and he's got permission, right? Right? Okay, so you didn't get the memo, but as long as you keep your head down and don't tell the co-founder when he asks if you've saw Tim Cook lifting design documents from the lab, nobody will think you're incompetent.
Of course, in Steve Jobs' day, people had to come to Cupertino and meet with him. But Tim is out amongst the people, wandering through Foxconn factories, foreign telecom headquarters, and Valve whenever he feels like it. Who's going to stop him?
Well yeah, somebody probably just saw a look-alike in the area and reported it as news to a bunch of rumor sites that spread it all over instead of double-checking the facts. Or, there are NDAs involved and nobody can admit to anything even though it's true.