If you're forking the project, sure. But I don't think that's what Gentoo is doing.

If you're trying to build what the project owner intended for you to build, I don't see how you can just ignore what the project owner specified. Nor do I see how that is allowed by the Python specs. That's why I asked you for a reference if you think it is in fact allowed by the Python specs.

If your position is that the Python specs don't allow you to just ignore pyproject.toml, but a distro can just ignore what's in the Python specs, I don't agree with the latter either, but it's a different position, and I'm not sure which one you're taking.