They say "mark" and not "trademark" in this takedown notice probably for a reason - because they can't call it a trademark and it's not enforceable?
If somebody creates ClosedAI company with product called ChatLLM, does it mean they can start sending takedown notices to everybody left and right who is using "LLM" in their name?
Those things are nothing alike.
GPT is a very specific family of models, all created by OpenAI. The copy-cats came after OpenAI released those models, specifically to point out similarities with the OpenAI-created models.
LLM is a generic term and cannot be trademarked.
You can trademark "Coca Cola", but you can't trademark "Cola" or "Soda".
If LLM is pizza then GPT is ie. thin crust pizza.
ps. it's funny how quickly they went from open ai research to prosecuting project for using gpt acronym while pumping money and doing exclusive deals with mega corp - the very thing they were suppose to protect us from.