While Non-compliant backends can do a lot, compliant ones can still add additional dependencies (Both buildtime and runtime), I don’t think pip or uv have a means of detecting a non-compliant backend outright replaced one package with a drop-in replacement that used native code, and even within compliant backends, that doesn’t mean that there aren’t other dependencies than what’s listed there that the backend could use, which would still lead to a differing list of actual dependencies, just in that case, that differing list is an extension of the original and dynamic, not discoverable without the backend.
nvidia-stub is a clever hack. It’s a backend, and you can unzip the wheel if you want to look at the specifics of it, I actually thought more people would be aware of what it is and does.
It results in modifying metadata during build, and downloading from https://pypi.nvidia.com during install
details about it have come up in packaging discussions before: Selecting variant wheels according to a semi-static specification - #16 by msarahan