The semantics are important. A release is composed of multiple distributions (sets of files). Each distribution does have a one-way hash value. This is what you lock to. Adding files does change the value of that hash. Files cannot be removed or changed.
You lock to the distribution instead of the release so your build doesn't have to download distributions for platforms you're not using.
Which is the entire problem. "Release" should be a signed hash of a manifest file that contains hashes of all the other files (in case of Git that's what commit does).
PyPi just decided to do what VCSes already did, worse