I find this puzzling. It does not seem to allow to stack PRs on top of other people's PRs?
There is already an option to enable review comments on individual commits (see the API endpoint here: https://docs.github.com/en/rest/guides/working-with-comments...). Self-stacking PRs seem redundant.
Still feels like an alpha version right now. I'm sure they will add it later.
Graphite (which they seem to be inspired by) has frozen branches exactly for that use case:
https://graphite.com/blog/introducing-frozen-branches
This API leaves a comment, on the commit; not quite the same thing since in GH, several operations are tied to PRs and not to commits.