Try doing some useful work after installing the Linux kernel – and just the kernel – on a device.
You can't. You need more than just a kernel, quite a bit more, and that's an operating system. "GNU plus Linux" or whatever you want to call it, or Android, or whatever. In that sense Stallman is correct that "the Linux kernel is just one piece of the operating system" (although his insistence on GNU/Linux is still rather wrongheaded IMO, but I don't feel like repeating that discussion).
There's plenty of people who do - see the https://en.wikipedia.org/wiki/Unikernel stuff and Firecracker VMs and simplified runtimes like Lambda - But agreed, a useful consumer computer requires a whole suite of programs and tools that comprise a "OS" to be useful.
the author considers an "operating system" as the ecosystem where the user live and find a convenient application "store". Same store = same OS. Different store = different OS.
So with this reasoning maybe soon Windows = Linux but Android != Linux.
this author has no clue what operative system is. There is more sense in the Stallmann GNU/linux copypasta.