Tabbing is not implemented exclusively by the WM, currently -- that's correct. But there exist window managers that handle tabbing. So, it's largely a matter of removing code. Lots of code. And then fixing more code that makes assumptions that it controls the drawing of the tabs.
And there's already the Xembed protocol for embedding windows in other windows, so it's technically possible to even move tabs from one application into another, with a coordinating dance. None of the changes that he wants really needs changes to X11 (although, as far as I know, it would be totally impossible under Wayland.)
It just needs someone to change applications to support it. I'd be interested to see an attempt.
considering how easy it is to switch tabs in a browser compared to a buried filesystem window, or multiple windows that are also buried. I can definitely see the use. I'd like to be able to have the file browser and terminal share a window via tabs as well.
I'm not sure we'll see it though, as for the most part the applications are developed fairly separately, and almost certainly won't see it in an open operating system working well for even most apps that people use. Short of maybe google releasing a filesystem browser extension and terminal extension, which may be entirely possible.
Sure many file-browsers (thunar, pcmanfm ...) can tab file-browser views, but I don't see the need for web-tabs in a not-web-browser. Firefox likewise can show folder content for file:///, but not feature complete compared to a file-browser.
And there's already the Xembed protocol for embedding windows in other windows, so it's technically possible to even move tabs from one application into another, with a coordinating dance. None of the changes that he wants really needs changes to X11 (although, as far as I know, it would be totally impossible under Wayland.)
It just needs someone to change applications to support it. I'd be interested to see an attempt.