It works for both. You can press Command+S in the message-list for senders to be outdented above. For implementors, just highlight the message in the code at the bottom and press Command+M. You can even select it "grossly" and there can even be intervening code in the middle of a keyword message and it will still work!
That bit? The selecting of a bunch of text around a keyword message? That impressed me no end. It _rocks_.
Oh I know, and I think Dan Ingalls is the one who originally implemented it over a decade ago!