The difference is that Linux isn't, and doesn't claim to be a whole system. For example, Android isn't Ubuntu, but Android and Ubuntu are equally Linux.
EDIT: I am not trying to make the "Linux is just a kernel" point here. Android is by no means a "traditional" distribution of Linux, but I just don't see why that makes it "not Linux". Busybox is available for it (Android is designed for embedded systems, after all) so what's the difference?
Yeah, "Linux is a kernel" and all that. Except, for many of us, Linux is also an ecosystem, and the Android ecosystem is definitely not that ecosystem.
The difference is that Linux isn't, and doesn't claim to be a whole system. For example, Android isn't Ubuntu, but Android and Ubuntu are equally Linux.
EDIT: I am not trying to make the "Linux is just a kernel" point here. Android is by no means a "traditional" distribution of Linux, but I just don't see why that makes it "not Linux". Busybox is available for it (Android is designed for embedded systems, after all) so what's the difference?