Yes, a branch together with information about the intended point of divergence is a stack of commits. (Most of the time, the point of divergence is arrived at implicitly by specifying the intended mother branch, and git figures out the last common ancestor, ie the merge-base.) Let's call the branch-plus-point-of-divergence a 'PR branch'.

Though your 'stack of commits' is more like a 'stack of diffs' (at least in the Git sense of commit-is-a-snapshot-of-the-whole-repo.)

And actually, for better or worse, a PR branch is more expressive than a 'stack of commits', exactly for the reason you suggest: a PR branch can also contain merge commits, it doesn't have to be a linear chain.