I'm Premier Platinum and got that offer this morning too. Just re-booked one of my upcoming reservations with this new PC code. Including insurance, the 2-day rental would be equivalent to buying UA miles straight up at 1.89cpm while getting a car rental for free. Excellent deal indeed.
The sad thing is that for that trip, I'll be earning ~3300 miles from the flying but a whopping 6250 miles from the car rental.