I don't think I do want to be pedantic but, to be explicitly clear, the philosphy I'm associating with uutils is capured by the "rewritie it in rust" mantra, so whether or not their posix compliant interface uses GNU code is beside the point.
More importantly, if their goal was a posix compliant api, then I'd have assumed they would have included that in their repo description, or even called the project posix.rs; but they didn't. What's more is that the rust standard library isn't equivalent to that of C, so it would still make sense that for a project whose goal was, or has simply become, to replace a C codebase would offer a safe C alternative to its dependencies. But maybe that's their next step; an internal library offering enough of a posix api to avoid surprises like these.