Every single centralized service (Hacker News included) reserves the right to ban anyone for any reason. Twitter may have a policy, but in practice it's no different than anyone else's Code of Conduct. It's certainly not legally binding.
Regular users don't care about whether someone could theoretically become abusive in the future - maybe they should care, but they don't. They just look at the track record and decide if they trust that person right now.
And right now, Mastadon instances are better at moderation than Twitter. You're worried about an instance admin theoretically not protecting you in the future; Twitter already, today, doesn't protect you.
A lot of people are OK with a middle ground where moderators remain, but are closely associated with their communities to reduce communication problems and increase their stake in doing a good job. Is it really crazy to believe that someone might want to trust other people, but also want to have more selection over who they trust?
You're here, so I would hazard a guess that you're OK with that middle ground too. HN admins could theoretically turn this site into 4Chan or Reddit if they wanted to. But we both trust them not to, and the fact that they theoretically have the power is not enough to make either of us throw up our hands and say, "well there's no difference, and nobody will care, and there's no reason for anybody post on HN instead of Reddit."
There are a lot of people here posting on a dedicated tech-community Reddit clone about why fringe Twitter is a bad idea.