This is very clearly an attempt by X to use the legal system to harass people doing perfectly reasonable things, but are you sure they are legal? I would assume that is against the ToS and I don't think the law makes an exception for cases like this (IANAL of course).
Whats illegal about it? The one vague Texas law that was in the original C&D? Everything else was just TOS violation.
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Terms of service have no legal binding unless both parties agree to them.
In other words, if X has a public API and Nitter is using it, then neither Nitter themselves nor the Nitter users have to agree to any terms of service in order to do so.
X cannot expect anyone to behave in accordance with terms and conditions they never agreed to.