Gruber has always been wrong about Mail.app facilitating top-posting. As long as I can remember, anyways. What Mail.app and other e-mail apps do is the right thing for people who care to reply in the correct manner: quote only the relevant parts. Going through the original message, in its quoted form, and deleting the irrelevant parts is just way easier than using two windows, one with the original message and one with your reply, and copying over the relevant parts you want to quote, always being careful to 'paste as quote'. And because you're going to do that starting at the beginning and stopping at the end (duh), Mail.app quotes the entire message and puts your cursor on top.

Clueless people can abuse this set-up for top-posting. And for some users, quoting anything at all is an exception, so they'd like to have the option of getting a blank reply window, instead of one from which they have to delete the quote first. But in general, what Mail.app does is the right thing to do, and it is not there to facilitate top-posting. Maybe it has this effect in practice, but the solution to that would be to educate people about proper form and why it's the right choice (which Gruber does, I'm grateful for that); the solution shouldn't be making the app more awkward to use for most people.