They registered “hordr”, not “hoarder”. It’s not your fault that there exist valid words in the dictionary, that describe what your app is doing, that they are not using.
This is just the usual case of domain and trademark squatting. If they attempt to further raise a finger (which from what I have read, from a judiciary point of view they haven’t), you have good grounds to countersue. You can also provide the C&D as evidence of threatening and harassment and probably counts for suing the party who sent it if they used a third party, as there’s supposedly a penalty for issuing false or trolling C&Ds.
That said: in a decent legal system no one should be able to trademark dictionary words. I’d suggest you change your trademark from “hoarder” to “hoarder.app” or something similar, as at the moment you trying to trademark a dictionary word is a vulnerability point that opponents with more money to waste can use to attack you, as this shows.
Ewwwww, it uses AI!
Imagine wanting to support something as efficient and energy-friendly as SQLite, then throwing an AI on top of it.