No - it specifies the serialisation format for
names/specifiers/extras/markers that is in common use, but doesn't
specify a programming API. It is intended as an interop building
block,

I'm not not talking about programming API.
this PEP would set the format used in interop formats, as you say, but also doesn't it effectively standardize the format used in the pip UI?
or maybe it's just that pip could easily say..  "our syntax for dependencies mirrors PEPXX exactly"