This is very much false, Primary sources only play a supporting role on Wikipedia, but they are definitely allowed. For example, if you're writing an article on Apple you can cite Apple for what Wikipedia calls "uncontroversial self-description". However, before that, you have to establish the notability of Apple through reliable secondary independent sources. The contents and focus of articles is also dictated by secondary sources. For example, if you take a controversial subject like Urbit, the article would have to reflect the priorities of (mostly critical) journalistic pieces on Urbit. You can cite its documentation for a technical description (that would be "uncontroversial self-description", as I mentioned before), but this would have to be a small part of the article, because it wouldn't reflect the focus of secondary sources.

Which is often stupid when the only people who know the truth are the people who were there. Hearsay from secondary sources is not an improvement in that case.

That’s why I used to like Quora - you would often see an answer provided by the primary (and only definitive) source for questions.