In a lot of projects topic branches generally have a single owner and are not used as the basis of other people's branches, even if they are technically public they are not public in the same sense that he is referring to. If you aren't going to be getting PRs on your branch you can consider it private and rebase all you like IMO.

edit to add: I generally prefer people not rebase after they've asked for a PR review just because the reference for comments will be lost. If they want to, maybe do it after all the reviews are approved.