So is Android not an OS because it uses the Linux kernel? If it is a program that manages other programs and is itself not managed then it is an OS, is what I've heard. Just because you're only making a small part yourself doesn't make the assembled thing not an OS.
Android is not distro, it is a fork. In similar way they could build a new os based on linux but the work compared to just replacing UI is massive and maintenance is absolutely not something you want to do if you just want to replace ui.
