What makes you think they only do this on Reddit? Surely since they don't disclose this one on Reddit, that means they wouldn't disclose here if they did the same with HN. Even if it's not them, surely the idea isn't unique enough that nobody else might do this.
You should absolutely assume that people are running bots on hackernews. The technology exists. I don't think it would even be that hard. I expect I could have a functional prototype in an hour (puppeteer to run a browser, simple Markov Chain to guide actions through states, hit OpenAI's endpoints for completions using OP's prompt slightly modified for hn). Long term, you could refine this with a locally running model that you fine tuned on comments (plus measures to avoid bot detection).
The comments made by the experiement are no better or worse than standard comments you'd find on those threads anyway. That old people collecting things comment was actually pretty good.