There is a connection with PEP 517 and projects that can't install via wheels too.
The thing breaking install_requires is actually the install-by-wheel feature of pip (not pyproject.toml as I initially thought): older versions of pip used to install packages in the correct order as specified by install_requires (this is actually documented).
Newer versions that install by wheels no longer do this: wheels are built in a random order and then installed. (this doesn’t break anything for the moment because pip still falls back to the classic installation if building a wheel fails)