I'm using an iPhone and no such luck.
Maybe it's just a referral for first time users. Did you click on it?
Offer just appeared tonight! Woohoo 
Doesn't seem like iPhone app can request uber from within the app (without redirecting to Uber)
You can with the latest version of the app.
Data Point:
I have an iPhone, my wife has an Android and we are both existing users on Uber.
We both recently spotted the offer in the cab section of our Google Maps App.
We both requested an Uber via Google Maps and were not redirected to the Uber App, although we were able to view our rides in the Uber App like regular.
The prices that Google Maps quoted us already included the $15 promotional discount.
When we opened up Uber, which was already in sync with our ride, the pre discounted price was showing.
Upon completion of the ride, the promotion was applied and brought the price down to what Google Maps originally quoted us.
Boruch Hashem we both received $15 off of our ride!
PS- The offer finally appeared in Google Maps (coincidentally) shortly after I ran the updates for my apps. My guess is that my Google Maps App wasn't updated originally and got updated and then offered me the promotion.
Also, look here at what Uber is officially saying:
https://newsroom.uber.com/googlemaps/"For a limited time, all new riders in the U.S. will receive $15 off their first ride."
Do they mean new Google Maps riders? Or is it just a quirk that existing users are also getting the promotion?
UPDATE: I just reviewed my receipt via Email and it confirms on the top that this is a "new and existing user promo for deep GMM integration" see snapshot:
