Sumary: it's a copy of Wikipedia, but without the bits Musk disagrees with. Perhaps it also has some other sources, but the articles I looked up when it was announced, were verbatim copies from Wikipedia, with some bits missing. My suspicion was that they instructed their LLM ("Grok". I wonder why it wasn't called X or Grox. Anyway...) to synthesize the article from the edits, leaving out those that were rated "liberal", with the error margin on the conservative side (pun intended).