Don't get me wrong, I love `uv` -- I mean as far as being "the backbone of any modern Python library" -- shouldn't any modern python library work with any packing system that follows the PEPs?

It depends on what you mean. The PEPs are essentially a behavioral core, and each tool (build backend, etc.) adds on top of that core. If you’re installing a package, any installer should work; if you’re developing a package, you may be subject to implementation details from your development tool of choice.