The way this is explained is like it hasn't occurred to them that 'create a branch' doesn't have to mean '...off master' - I continue working with a new branch that depends on the old thing that's in a PR still simply by basing it on top - but if this comes from Google and Meta and is popular, obviously that's not the case.
My guess is that it's more about somehow seeing a more 'contextual' (not semantic exactly) diff in the second PR? So rather than including all of PR 1's changes that it's based on, just showing the actual diffs in the commits in isolation, or as if rebased (only PR 2's intended patch set, not the master fork point through tip of PR 1) on master?
Though I haven't read further than I can as a non-subscriber, perhaps it would be clearer when it explains how they're doing it on GitHub (the above would require some rebasing or target branch and further PR automation).
My guess is that it's more about somehow seeing a more 'contextual' (not semantic exactly) diff in the second PR? So rather than including all of PR 1's changes that it's based on, just showing the actual diffs in the commits in isolation, or as if rebased (only PR 2's intended patch set, not the master fork point through tip of PR 1) on master?
Though I haven't read further than I can as a non-subscriber, perhaps it would be clearer when it explains how they're doing it on GitHub (the above would require some rebasing or target branch and further PR automation).