You seem to have sufficient reasons to reject the paper on its (de)merits, whether or not it was written by a bot.
You can certainly tell the editor that you are suspicious.
I doubt that the editor has software (free or otherwise) that could tell whether the paper was bot generated.
I wonder if the many citations are real. Chatbots are notably nonsensical in that realm. You could quickly check a few. Even if real they may be irrelevant.
Finally, LLMs can indeed scrape SE posts. Whether they could use that information to learn how to do better is unclear.