And almost as if on cue, my Dividend Miles account started showing a "DMS" notation overnight and now, when I attempt the same dummy booking as yesterday, I get the 5000-mile discount. So, from activation of my card on 10/12, I waited nine days until my new status posted.

Just in case this makes any difference, my first billing period closed on 10/16 so my status also did not change until after this event. I haven't paid the first bill yet so payment apparently does not trigger the status change.