If they wanted a trademark, that needed to appear on copy from day 0. The paper is titled: "Improving Language Understanding by Generative Pre-Training" and not "Improving Language Understanding by Generative Pre-Training (TM)"
They ALSO would have needed to use a different generic term to refer to the technique. For example, Velcro always says things like "Velcro-brand hook-and-loop," and ALWAYS uses a generic term to refer to hook-and-loop when talking across brands. It ONLY uses "Velcro(tm)" to talk about their product specifically. OpenAI started using GPT generically.
Publications like this one, if anything, undermine OpenAI's case since they're using the term generically.
The decision is correct. Trademark law is used precisely to prevent this sort of thing: OpenAI can have a generic term or a trademark. The law is set up precisely to prevent a company from building their brand value by retroactively trademarking a generic term once relying on a community to get it established.
To be more blunt: I use "GPT" generically because they encouraged me to do so. That should not contribute brand value to OpenAI GPT-3 or OpenAI GPT-4.