I'd probably buy donuts from your hypothetical closeted 9/11 conspiracy theorist. There's certainly a selection effect, but I think such people are extremely rare. I hardly ever ask people what their beliefs are, but they tell them to me anyway. Of course, I'm no different.
People are typically much more passionate about their unusual beliefs. Every young-earth creationist I know of, I've found out about their belief within an hour of meeting them. I've never had a friend, acquaintance, or coworker who I discovered to be a biblical literalist only after several months of interactions. These beliefs are so important to these people that they simply can't avoid expressing them.
Likewise, they have a hard time avoiding the secondary beliefs stemming from their conviction. It's eerie how similar their opinions are on gay marriage, women's rights, obscenity laws, circumcision, and teaching evolution in schools. I have no qualms avoiding interactions with these people.
A side note: I'm glad this article was buried. The replies to my comments have been quite civil and well-reasoned, but most of the other discussion in this thread has been unproductive to say the least.
As a case in point, from my personal interactions with Brendan, I can assure you that he is not the sort of person who will tell you all about his thoughts on gay marriage within an hour of meeting him. In fact, I don't know what those thoughts are (short of trying to guess based on the donation thing), and I've known him for years. Mitchell Baker has known him for even longer, and worked with him more closely, but see the next to last paragraph of https://blog.lizardwrangler.com/2014/03/26/building-a-global...
You seem to think that people who have deeply held beliefs spend all their time trying to foist those beliefs on others around them. That's true in some cases, not in others.