Specifically in this case, it seems the developer isn't doing it for free - their work is at least somewhat sponsored and looking at their site it seems to be their full-time work. I'm not diminishing their time and effort, but this isn't someone working late nights that is hoping their software gets its first 100 users.
>They said that after the user already threatened to do so.
There's a button on every public GitHub repo to fork it. If I say "I'm going to hit that button" (even in a particularly rude way) - is that really a threat? It is functionality built into not only the site, but also the culture and code licensing that underpins any open-source project. I struggle to see how anyone could threaten open-source code by saying they'll fork it. Go ahead - that was specifically designed into the license for any reason. It's so prevalent that it has its own button.
>If someone says “we’re going to fork this because we disagree with your maintenance of the project”, they probably don’t mean “I’m gonna click ‘fork’ on GitHub.”
What then is your expectation for what it means? By saying "we're going to fork this software" are they now starting a competing company? Making hardware products using it? Monetizing the pre-existing work? Selling subscriptions?
These are all examples that I read your comment as implying that aren't actually enumerated or described anywhere.
My expectation for someone saying "I don't like how you're running this project and will fork it" is _exactly_ that they'll hit the fork button and make their own project. That's why it's open-source, that's how open-source software works - that's what the button is for. Even if they then went and built an entirely competing business, around one minor change I specifically chose not to make - then what is the issue? The license allows it - and I chose the license.