I think it is conceptually the same but multiple PRs gives you the tools to manage the commits properly which GitHub is missing. You can't do the equivalent of `git rebase -i` in the GitHub UI to squash a fixup into a previous commit. Having each change in it's own PR enables that workflow using the existing GitHub UI.

Split into individual PRs, which works better for how a lot of companies do code review.

Stacked PRs track changes through updates and can be integrated progressively as they get validated.

They also allow reviewing commits individually, which is very frustrating to do without dedicated support (unless you devolve back to mailing list patch stacks).

Each commit can be merged independently as they're reviewed.

Commits are immutable and you never know which feedback goes stale when you add another commit.

I'm not a huge fan, since stacked PRs mean the underlying issues don't get addressed (reviews clearly taking too long, too much content in there), but it seems they want something that works for their customers, right now, as they work in real life.

It would be the same if commits were meaningful things. But typically they're not. The PR is the meaningful thing. A commit is something like "fix the lint error".

CI runs on each PR, you get a whole PR message and discussion/review interface for each PR. Each PR can itself consist of multiple commits. You can have stacked PRs from different authors (though from another comment it sounds like they may not have implemented that).

It's a big improvement (assuming they've done it right).