1) Substack links are blocked and restricted. I've personally confirmed this, as have countless others. In fact you can't even search for the word substack (or tweets made by Taibbi!) right now. Musk is either attempting some sort of deeply disenguous argument about the meaning of the word "blocked", or he's just mistaken. At best.
2) I can't even parse the nonsense about Substack scraping twitter. It makes no sense on multiple levels. There's no real way they're trying to download a "massive portion" of Twitter's content, there's no way they're "downloading the Twitter database", there's no way they only have a single IP address, and this block isn't about IP addresses, it's about domain names, or in the case of the search bans, actual words. And also, there's no reason why they'd want or need to do whatever he's accusing them of to start their Notes product. This is so garbled is hard to even call it wrong.
3) Taibbi isn't a Substack employee. Not that it'd particular matter if he was, but he's not.
I'm honestly at a loss. This is so goofy and obviously wrong it's hard to figure out if Musk just believes his target audience will happily swallow nonsense and so has no need to make up plausible lies, or if Musk actually believes this stuff.