To take advantage of this promo, all you need to do is buy the phone at the discounted price via Google Fi, then be sure to activate the device within 30 days. If you don’t activate it on Fi, you’ll be charged the full price of the phone. And yes, this promo is open to new and existing Fi customers.
Full terms can be seen here.
Did you know you can download the latest Android Q beta with the Pixel phones? That’s pretty sweet.