I agree with this, too. Again, preaching to the choir. But it does appear he looked it up. He just didn't look far enough. Google's overview says the capacity is indeed 20k, while Wikipedia (which Google claims to have cited) says it's 19k.
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There's a lot of problematic things to unpack with his tweet and his glorified blog post, but I think there's also a variety of other problematic things to account for in other places, starting with this nonsense from Google. At the end of the day, as I said, he pays for Twitter yet is unwilling to use the edit function he pays for.