I think the point is that if you have to squash, the PR-maker was already gitting wrong. They should have "squashed" on their end to one or more smaller, logically coherent commits, and then submitted that result.

It’s not “having to squash”. The intent was already for a PR to be a single commit. I could squash it on my end and merge by rebasing, but any alteration would then need to be force-pushed. So I don’t bother. I squash-merge when it’s ready and delete the branch.