Continue watching. You can hide Stage Manager like you can hide the Dock
Yes I saw it and I knew about this feature.
But first, when he recorded the video it was probably only half-true. I remember trying SM when it was first introduced, and at some point it shrank the app windows back to reveal SM even after you enlarged them manually. I tried it now and it appears that Apple fixed it by now. Windows remain at their full size (although they initially all shrink when you activate SM).
Second, this doesn't work in many Adobe apps (SM doesn't appear when you move the mouse to the screen side). It doesn't work in InDesign, Photoshop, Illustrator, and Dreamweaver - and who the hell knows what other apps it doesn't work with.
Third, this auto-reveal feature is kinda finicky. If you move your mouse beyond certain point (even if it's still over a SM window), SM can disappear on you and you need to reveal it again.
Fourth, all use-cases in the video are based on SM being revealed anyway, which reduced the usable screen estate dramatically.
So yeah I personally still don't find SM useful for anything whether it's revealed or hidden.
However, I do realize that some people find it useful and if it does help someone then obviously I can only be happy. Sure Stage Manager maybe annoys me more than it should lol.