If it’s using a virtual environment, then they can pierce the abstraction by inheriting system packages, including a compatible uv-build. There’s nothing about Python packaging that requires them to rebuild the backend multiple times.
Which defeats the purpose of doing the build in a virtualenv, which is to avoid any possible collision with or contamination by other stuff that's installed on the system by using a fresh copy of everything.
Speaking as someone who has written many Python packaging PEPs and who has maintained (and continues to maintain) PyPA tooling that you almost certainly use: uv and uv-build are as much a part of Python packaging as anything else that interfaces with the standards.