Why would I care that the maintainer prefers Mercurial? The document you linked to doesn't say anything about them not taking community submissions: I think you need to scroll down further in the page. And it certainly doesn't say that you have to use SO to submit changes.
refuses to use git [0]
refuses to take community submissions (except through Stack Overflow? Seems like a misuse of SO) [1]
and refuses to implement .dumps() [2].
He is difficult to work with, and any time I need to debug code that intimately deals with ruamel.yaml types, I wince.
[0] https://github.com/pycontribs/ruyaml/issues/1
[1] https://yaml.readthedocs.io/en/latest/contributing.html
[2] https://stackoverflow.com/a/63179923/15170511