The PEP contains a section “Adding sys.abi_features to the Python Standard Library”, which requires a core PEP.
You may need to split into two PEPs to enable the packaging side of it. Though it’s possible that a PEP could be skipped if it’s literally just going to be to enable x in sys_abi_features on versions of the runtime where the sys attribute exists.
(And just to be 100% clear, I think the post was and the PEP is correctly classified, it just has to talk less about environment markers as specification and move them to examples of how the new sys.abi_features attribute might be used.)