Pip can use range requests to fetch just a part of the wheel, and lift the metadata out of that. So it can sometimes avoid downloading the entire wheel just to get the deps. Some package servers don't support this though.
Also, there's a difference between a pep being adopted and that pep being implemented (usually a bunch of elbow grease). That said there are a couple exciting steps towards 658 being implemented:
https://github.com/pypa/pip/pull/11111 (just approved yesterday, not yet merged)
https://github.com/pypi/warehouse/issues/8254 (been open for forever, but there has been incremental progress made. Warehouse seems to not attract the same amount of contribution as pip)
Also, there's a difference between a pep being adopted and that pep being implemented (usually a bunch of elbow grease). That said there are a couple exciting steps towards 658 being implemented: https://github.com/pypa/pip/pull/11111 (just approved yesterday, not yet merged) https://github.com/pypi/warehouse/issues/8254 (been open for forever, but there has been incremental progress made. Warehouse seems to not attract the same amount of contribution as pip)