I’ve expanded my answer on the other tread (No Dual-ISO anymore? - #9 by Alevesque).
Basically, rest assured you are still getting “dual-ISO”, and then some. The 400 and 1600 native ISOs are what we refer to as dual-ISO, but the interesting part is that the 640, 800 and 1250 are equally native and essentially offer additional steps to get the most out of the sensor.
Should we get the quintuple-ISO moniker going?