I think it does require a paid account, Hydroxide basically acts like the official Proton bridge.
I haven’t actually tested with a free account, so there’s a chance it does work. When you run the auth command (which is the same as upstream Hydroxide), it will probably throw an error.
If you have a free account and try this out (or Hydroxide), please report how it goes back here, I’ll add a note to the readme. Upstream doesn’t seem to mention this in their repo either.