Looking forward to playing this, pre-ordered the Ultimate edition a couple of days ago.
Can't say I understand or agree with their stance of sending out access to the Definitive Edition in waves "from" today though - why not just unlock it for all pre-orders at the same time, especially if it's already registered to buyers' Microsoft accounts, which it seems to be (shows up in my Manage content for Halo Wars 2).
If I were to pre-order a game it wouldn't say "you'll get your code at some point after release, we send them a few at a time", so I don't understand what's different here.
You have to understand man, it takes time to send out the codes to all those people who pre ordered the digital ultimate edition, there are dozens and dozens out there.