Those are refs. Not commits. GitHub removes access to the old commits of a branch when the branch is force-pushed (say, after a rebase). I'm interested in comparing the branch in its current state against it's previous state.
tl;dr you can put commit hashes in the place where the parent comment put tags, and your exact use case actually has a dedicated link on the PR page.
The PR interface actually exposes this for force-pushes, but the UI discovery for this is horrible. It turns out that the "force-pushed" part in the little message in the github UI is actually a link. This link points to the diff between the old and the new HEAD of the branch.
Pressing the "force-pushed" link works fine for me on private repos too. It could be that they fixed this recently, I only found out about this feature ~a month ago. If it still does not work on private repos for you, maybe try reaching out for support. It could be that there's some bug you're hitting.
