Nobody said anything about "published" sources. Anything that is not Wikipedia (or another purely-editorial tertiary source like a rival Encyclopedia, or a survey-level textbook) qualifies as a secondary source.

Assertions about history before the Internet—before writing, even—can be cited just like anything else. The "99% of history" that we currently know about, we know about because some historian or paleontologist or anthropologist dredged up primary-source data, put it together, made sense of it, and wrote down their findings in a journal paper (i.e. a secondary source.) Wikipedia, like all encyclopedias, cites those secondary sources.

You have primary-source knowledge of your own? Write a blog post about it. Just like a historian writing a journal paper, that blog post is now a secondary source that quotes your primary-source knowledge (ETA: or maybe the blog post is even a primary source itself, depending on how fresh your knowledge was and how unbiased your reporting of it was.) Wikipedia can now cite your blog post. Wikipedia cites plenty of blog posts.

Alternatively, if you're doing journalism—going out and talking to primary sources—then the Wikimedia group of sites has a place that will host any secondary-source artifact you create from that: Wikinews. You can write a Wikinews article, and then cite it on Wikipedia. Wikipedia cites plenty of Wikinews articles.

Let me make another analogy: if you compare the Wikimedia foundation as a whole to, say, a newspaper publisher; then Wikipedia is specifically the editorials section of that newspaper. Journalism is most of a newspaper; but the one place it doesn't belong, is in the editorials section of the newspaper. Journalism belongs in the "news" part of the paper.

Wikipedia editors know that people can wear many hats, and there's nothing wrong with being both an editor and a journalist. The whole distinction they're making, is that Wikipedia doesn't want people editing articles with their primary-source hats on, or with their journalist hats on. Wikipedia wants people editing articles purely with their editor hats on. If you're a primary source, or a journalist, you do that stuff outside of Wikipedia proper. Then you turn around, take off those hats, put on your editor hat, and edit Wikipedia to refer to what primary-source-you or journalist-you just created. (Or, if you're not an editor by nature but just want your content cited, then you should just be a primary source and/or journalist—someone who writes stuff down somewhere it can be cited—and leave editing Wikipedia to people who like editing. Maybe make friends with some Wikipedia editors, and send them links to your new stuff when you produce it. If it's worth including, they'll cite it and write about it! This is literally exactly the same as having a relationship with the editors of a newspaper/magazine/regular encyclopedia. Just because you/anyone can be a Wikipedia editor, doesn't mean that you have to; and it doesn't mean that being a (Wikipedia) editor is not a specialized job that is some people's comparative advantage, while your comparative advantage might lie elsewhere—like in being a personal historian.)

I want to call out a specific example of blog posts being cited as secondary sources, to make this clear. Microsoft's Raymond Chen is a blogger (https://devblogs.microsoft.com/oldnewthing/). He writes a lot about the path-dependencies in Microsoft products that have led to them being the way they are today. Wikipedia cites these posts all the time. Even though he's writing about things he did himself. You are allowed to be a primary-source man-on-the-spot recollecting history; and the secondary-source historian "interviewing" the primary source; and even the tertiary-source editor citing the secondary-source blog-post "interview" artifact. (I don't think Chen bothers to edit Wikipedia to cite his posts, but there's no reason he couldn't.)