> I'm confused, I thought the ability to install apps outside of Play is supported by the OS.
It is.
What this is doing is forbidding apps that can install other apps from being in the Play store. Basically, if Total Commander wanted to give the finger to Google and refuse to remove the function, Google would remove them from the Play store, meaning you'd likely have to either use your browser to find and download the APK or install it over USB with ADB.