I'm not quite sure this follows. At the very least, I think they should also consider the possibility of social contagion: if some of your colleagues start using a new word in work-related writing, you usually pick that up. The spread of "delve" was certainly bootstrapped by ChatGPT, but I'm not sure that the use of LLMs is the only possible explanation for its growing popularity.
Even in the pre-ChatGPT days, it was common for a new term to come out of nowhere and then spread like a wildfire in formal writing. "Utilize" for "use", etc.