On the other hand these days[1] we run OSs within OSs via virtualization. And we can run apps on bare or virtualized metal without an OS. So the I think that the distinction between app platform and OS is almost non-existent. Given virtualized hardware, even OS as an hardware abstraction client is not so clear cut.

I'll say it: the browser is an OS, sue me.

[1] we always could of course, it was just not mainstream