In my opinion, this is a very good move. The biggest, most gas guzzling vehicles should be made more efficient (whether that is deisel, gas-electric, or whatever) before the small ones.
20/21 versus 12/18 is a huge fractional umprovement for city mileage. The 2008 camry for example is 19/28 (V6) and its hybrid equivelant gets 33/34. That is also a great improvement but see below.
So on a theoretical 1000 mile month in the city you save 83-50=33 gallons with the escalade, and 53-30=23 gallons with the camry. So wouldn't reducing the consumption of the largest vehicles first make most sense? Al consider that the hybrid escalade is rated better in the city than the non-hybrid camry.
On the flip side, given there are many more Camry's than Escalades out there would petition for making the Camry more efficient before the Escalade. Either way progress like this is good.