Back in the 80s and 90s people did not often distinguish between app platforms and operating systems, because back then there were no widely-used portable app platforms like Java or the Web.
Nowadays though, with a few popular app platforms running on numerous operating systems, there is a clear distinction between OS and app platform.
Emacs is an app platform for client-side applications.
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
