The polite thing to do is to check in on the status, in a public setting, and get an answer from the maintainer, which is what this person is doing. Yes, it's entirely possible that there's nothing new to add to it, and it's "done." But what if there is somehow some sort of major change to OpenAI's API, or they deprecate [current API version] and you're expected to migrate to a new one? Or, worse still, a major exploit is found? These are valid concerns that even free users would want to know, and the best way to do that is to check the pulse of the project.
The person in the screenshot made extra care to be polite about asking, and they're now being put on blast for it. Also worth mentioning is the user who commented saying there are "relevant and important" PRs and issues that haven't been addressed over the last year. Whether they're relevant and important, I don't know, based off my experience I'll say likely not... But the fact that you're allowing irrelevant and non-important things to go stale, without closing them, only lends credence to the possibility that the project is abandoned and, if something were to go wrong in the future, nobody would be around to fix it.
Also, your statement that they're offering to fork it as a "threat" against your project is not at all what is going on. If your project got forked with reason "original was abandoned," I think you'd be even more frustrated than you are now. They asked what the status was before putting in the work to fork it and attempt to move users away from your project. That is a courtesy notice, not a threat, designed to save everyone time and frustration.
It is a good thing for someone's open-source code to have interest by the community, and for users to want to use it. Getting angry at those same users for checking on the status won't do any good.