That seems like a likely explanation, probably won't get into legal trouble for using an OpenAI model for a research paper but redistributing said model may be upsetting enough for OpenAI trigger a legal challenge.

Unnatural language used davinci-002 although that was a while ago, they only say "similarly" in this paper and don't specify what they used. I can't see a reason why they wouldn't be releasing it if the unnatural prompts were generated by LLaMA2-family.

In any case, replicating this training seems trivial and very cheap compute-wise for anyone who wanted to do it.