Ah yeah, you're right, that's a misrepresentation on my part - it's based on lines, not the file:

> [absorb] splits changes in the source revision and moves each change to the closest mutable ancestor where the corresponding lines were modified last. If the destination revision cannot be determined unambiguously, the change will be left in the source revision.

I use absorb fairly often, fwiw. It's great for when I want to make a patch to a commit that will easily absorb into its right place. And I also, sometimes, prefer the more intentional approach where I decide exactly where each hunk will go.