IANAL but unfortunately the law isn't really in Nitter's favor, as Elon could easily win the fight that Nitter can be considered as a read-only scrapping mirror, which itself makes total sense why Elon tries to C&D it since it is a loophole for other AI companies, probably especially OpenAI, to read the feeds for free.

tl;dr Elon's ground can easily be that there is an official paid service that let's you read the feeds unrestricted (this will be a good counter-challenge however IMO) while Nitter tried to run around it. So the First Amendment defense from Nitter should be shit outta luck