Writing tools that are POSIX compatible doesn't mean one puts it on the pedestal of the "holy grail of OS design." I've certainly used POSIX to guide design
aspects of things I build. Not because I think POSIX is the best. In fact, I think it's fucking awful and I very much dislike how some people use it as a hammer to whinge about portability. But POSIX
is ubiquitous. So if you want your users to have less friction, you can't really ignore it.
And by the way, Rust didn't invent this "rewrite old software" idea. GNU did it long before Rust programmers did.