We should ask for links to verifiable 3rd-party websites.
Anyone can claim to be a relevant notable figure; StackExchange has no method of verifying these users, the way Twitter does. Because we can't verify such claims, they are essentially hearsay, i.e. opinion.
That said, notable figures can still provide answers. If you really are That One Guy, then you can post on That One Guy's blog saying, "I recently saw a question about X over on RPG Stack Exchange. Man, what a crazy game that was! Here's what really happened: ..." You can then answer the question by linking to the blog post, which is a verifiable communication from the person in question.
This approach benefits the stack in two ways: First, it makes these answers trustworthy, rather than just, "Well, some guy on the Internet claims that he was really there and it was X." Second, it helps drive additional traffic to the stack (assuming the notable figure actually references RPG.SE).
If someone wants to go the trouble of making a blog where they pretend to be That One Guy just to spread fake information or get a sock puppet some rep, that's a problem for That One Guy (and his lawyers) rather than for the stack.
To put this a little bit differently, who posted an answer shouldn't matter; the content of the answer needs to stand on its own merits. When someone answers a rules question by claiming that the rules work a certain way, we ask them to cite the relevant rules (in a quote and/or with a link) to properly support their answer. Answers to questions about specific real-world events should be held to the same standard. An answer that starts with, "I'm Bob and I was actually there," is ultimately no better supported than an answer that starts with, "I heard a rumor that..."
Just like we would ask someone who answered a rules question to properly support their answer so it can stand on its own, we should ask people who answer anecodotal questions to properly support their answer so it can stand on its own.