The key you get for the digital copy is the only key you should get/will get.
The physical copy is a DRM free physical copy (not attached to Steam), and that counts as your 2nd game copy. If you were to get a 2nd Steam key, that would count as a 3rd copy....
Edit: the mixup here is that I think the mail was referencing people who only had physical copy packages, which were being delayed shipping until after the release date...the so the digital key was for them so they could play on release day.
So people who ordered a physical copy get a Steam key (to download early) and a physical copy. According to you, 2 copies.
I spent $65 on a retail pack, and I spent $20 EXTRA to get additional digital key for my wife because I wanted two copies on release day.
What did that $20 extra buy me?