If Google can move all it's app and even core features of android like system webview component to playstore and even update them using google play services without updating device firmware which is in sharp contrast to Windows or iOS in which whole OS upgrade is necessary to update apps like browser, maps etc then I wonder why cannot they push Doze in playstore.
Anyway what google did to save battery life to those devices which runs stock android but still having Kitkat + lollipop and have not been updated to android 6+ or all the 44% kitkat and lollipop devices are from manufacturers who modify android and there is no stock android devices which runs android 5 or below ?