The whole point of Gentoo is to build everything directly from source. That means doing whatever the build source tells you to do. It also means not using a pre-built wheel, since that's not source code.
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.