If you don't enforce your copyright or trademark, you lose it. That's likely what this is: Some lawyer saw that DoomRL was using Zenimax property and got all excited about it.
The use of the umbrella term "intellectual property" is a deliberate tactic to seed that type of misinformation in public discourse, so that people will assume these different concepts are equivalent and that protections applying to one of them apply to all of them. You're seeing cases where this tactic has succeeded.