The Nix maintainers asked very nicely whether there was a specific reason and whether they could work with the author to solved it. The author just said "no, just don't include my stuff", which he technically didn't have the right to say after releasing the code as FOSS.

You can't demand respect from the Nix folks after disrespecting them by telling them to do something "just because".

If you follow the thread all the way down (and elsewhere, e.g. HA & Fedora communities), you'll see that the author does not seem to truly hold to his own statement that he "cannot stop [them]", and proceeds to attempt to bully them into compliance (despite many efforts by them to come to a compromise that may suit him)