(But also: if we’re talking virtual environments then we’re firmly in Python land, not distro land, where the correct answer is “use a wheel” instead of building it yourself.)
From what I can gather, Gentoo's Python build system does use a virtualenv [1]. Maybe that's not your definition of "distro land", but it appears to be Gentoo's.
It's not clear to me exactly what frontend build tool is used, whether it's one of the well known ones or something that the Gentoo developers coded themselves. However, if it's using a virtualenv, it's going to have to re-fetch the build backend and rebuild it.
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.