Eh, I don’t think that’s actually true.
The section you’re pointing to was updated without a PEP, to make previously compliant behavior no longer compliant, and it was called out by the author of that change 9mo later that he realized he updated that section of the PEP without anyone really discussing it because everyone focused on the version number and where the spec should live.
It was also pointed out that you currently can’t follow the updated spec and release to PyPI, because warehouse will reject your upload (pypi/warehouse#10030), which is still true today.
That was followed up by @pf_moore saying he’d rather we fixed Warehouse than go back onto the PEP, but that doesn’t seem like there was ever consensus for changing the rules to require normalization of distribution names, it just slipped in accidentally and has been in limbo since then.