The Android kernel has some changes that weren't upstreamed, has features that Linux doesn't, and is missing some features, as well. For example, SysV IPC isn't supported on Android's Linux kernel[1].

Often, for specific devices, the kernels they run are forks with changes that aren't upstreamed. Some of those changes can be significant enough that porting those changes to an updated kernel snapshot is nearly impossible, so it never happens, despite the kernel fork's source being available under the GPLv2.

[1] https://android.googlesource.com/platform/ndk/+/4e159d95ebf2...

Yes, my router is Linux, and my TV is also linux (containers not quite, because they don't emulate the kernel, except for Windows and macOS machines, that NEED to emulate at least one kernel instance, but whatever). And is pretty clear to me those limitations. More than my router, or my TV, or any embedded system for that matter, Android does try to follow other systems specifications, it has a own libc implementation, you can still utilize most of linux that we are used of: filesystems, syscalls, networking, even containerization (all of them limited and various levels, of course).

That is what makes a linux distribution too me in my opinion, in the same way I can run a linux with musl libc, and busybox, I can run Android