All right, here is sort of a rundown of what Musk has been tweeting vis-a-vis Pizzagate.
First, in response to a tweet claiming that Media Matters founder David Brock was connected to the owner of "the Pizzagate restaurant", Elon
tweeted "Weird."
Soon after, Musk tweeted a meme image containing text that asserted Pizzagate was a real child-slavery ring and claimed that an "expert" who had debunked Pizzagate, had just been convicted on child porn charges. In a reply to his own tweet, Musk further tweeted a link to a news article about the conviction. I can't link to either of those tweets because he subsequently deleted them.
Here is the article that Musk linked to, however. It indeed accurately reports that James Meek, an investigative producer for ABC News, pleaded guilty to child porn charges.
However, the James Meek in this article never wrote a book debunking Pizzagate. Musk seems to have mistaken James Gordon Meek, the convicted ABC producer, with James Meek (no middle name), a British author who contributed an article to the London Review of Books in 2020
about conspiracy theorists and theories, in which he mentions Pizzagate and QAnon among others.
Musk is something of a credulous moron who uncritically believes and regurgitates anything he reads that casts people he doesn't like in a bad light, and also loves to repost memes he finds in other places, so it's unlikely that this conflation error was his own original mistake - although I think it's a decent possibility that someone eventually made him aware of the mistaken identity and that's the reason he deleted those tweets.
But in answer to those of you wondering why Elon is suddenly stumping for the reality of Pizzagate now, the answer is that he is now invested in the conspiracy theory being true because it was suggested to him as another potential avenue for attacking/smearing Media Matters for whom he was already sporting a hate-boner.