The classic feature phones is an older Symbian variant and Asha Platform OS is S40 from what I can tell. Symbian is still king in a lot of countries so Nokia will have to support it for a long time.
Struggle to see how this helps Nokia. Nokia Xs are smart phones at a price of a feature phone. Great! Welcome to an already very crowded low-margin market of cheap Androids in India, Brazil, China and Africa, the same market MS will enter with cheap 8.1 devices and where Firefox OS devices are heading. Yet, Nokia Xs have little extra to offer to the consumers that would significantly differentiate themselves from the competition.