My issue with requiring a so-called "quibbler" to develop the proposed change themselves is it puts a high cost on making constructive recommendations.

This can be useful for dealing with the type of argumentative person who will find something wrong with anything. But for a busy person who's been assigned a PR to review, this methodology restricts them from offering feedback since they don't have time to develop fixes, only to suggest them.

Do you resolve this by ensuring PR reviewers are allocated enough time in their schedule to develop suggested changes?

There's another problem here. Requiring others to fix bad code instead of pushing back bad PRs to the original developer removes an incentive for them to write good code. If someone else will fix it, why bother? Do you resolve this at the performance review level?

There ought to be some middle ground here, where you can shut down actual quibblers while allowing legitimate feedback to be quickly given.