I create my GitHub PRs like a reviewer's going to look at the commits if it's too large overall. I'm also fairly sure they don't, because it's basically never worth doing so in their PRs ('fix the test', 'merge origin/master', 'address review comment', etc. commits).

I suppose I agree GitHub doesn't help with this / implicitly opposes it (I don't see the 'explicitly and' claim justified though?) but there's nothing about a PR that isn't a 'series' of 'patches'. Maybe GitHub's just responding to the way most people use it, and making that easier/better instead of being principled? Doesn't mean we can't be.