At least in Europe, there's an argument to be made about a Nitter instance being an interoperability service for clients that do not use cookies/JavaScript, and could be argued that it's protected under the interoperability exception under EU directive on legal protection of computer programs.

The directive gives quite wide exemptions for reverse engineering, but as there's no reverse engineering here, just documenting public APIs, codifying and interfacing with them, it seems it is more than OK for the compatibility layer to exist.