When you receive a request that is titled the "Nightmare letter" [1] it is hard to assume otherwise.

1: https://www.linkedin.com/pulse/nightmare-letter-subject-acce...

I don't think the TITLE of the letter is literally "Nightmare letter". It's the name of the letter that people copy and send.

I didn't read the whole thing, but the part where the troll says he believes the site is in violation and makes threats but doesn't have any actual proof that there is a violation sounds like a good indication.