I actually like the ideas they have here! It feels inspired by the best parts of macOS, Windows, and tiling window managers, while actually being pretty novel IMO.
However, I actually like the "overlapping sheets on a desktop" metaphor because it's still pretty "task" based. Their example is a little contrived, having a lot of small unrelated windows smushed into the middle of the screen is rare I think. At least for me. A more common example I think might be:
You're in a web browser full screen, and then you need to drag and drop a file into a site, so you open finder/explorer. That appears overtop of the browser. Then maybe you realize you want to remove some metadata from the file you need to upload, so you go into that file's preferences pane, which is a whole new "sheet" on your desktop.
At any point, you can dig up the browser, since it still occupies a lot of the screen. Effectively it's a branching undo tree. Since you connect all of these apps together manually (opening, clicking, dragging, sharing in MacOS), you're free to go and fiddle with things that are going to be accepting something else from another window.
The thing I think is missing from that metaphor is that it's really hard to get back to what you were doing if you break that undo tree. You click the browser, but now all of your other windows are behind, and the whole stack won't come back at once.
If windows recovered in the stack they were opened in when you focus them, (in my contrived example, you refocus the file preferences window, OR maybe there's some gesture/keycombo to return to the last focus state) I think that'd make a pretty measurable effect on my window organization annoyances.