The Retold pre-order FAQ page says:
Already own Age of Mythology: Extended Edition on Steam? You can get a discount!
Just make sure to sign into Steam with an account that owns Age of Mythology: Extended Edition and the discount will show up in the store.
I own EE but I can’t see a discount, other than a “special promotion” for the Premium Edition.
Is the special promotion the discount? It doesn’t sound like that’s what it is, and the FAQ doesn’t say the discount is only for Premium.