Alternative packaging toolchains can use pyproject.toml that way; it's an extensible file. But you've caught on to the key thing, which is that neither they, n

Alternative packaging toolchains can use pyproject.toml that way; it's an extensible file. But you've caught on to the key thing, which is that neither they, nor the default setuptools-based toolchain, have to do that. The pyproject.toml file exists to solve the chicken-and-egg problem of specifying that you use an alternative toolchain, when figuring that out might otherwise be dependent on your preferred toolchain already being present and configured.