It’s also allowing 8 apps to draw on screen at one time. (Not to mention that it’s holding those other apps on the side in memory for as long as it has enough resources to do so.) It’s the memory management that makes all that possible, the changes to UIView and to the window manager, as well as all the other supporting technologies (like changes to drag and drop). It’s not just a view transition, and, as a developer myself, I’m surprised you as a developer would boil it down to such.
I don’t want it to sound like a personal attack, but I’m legitimately curious about your educational and professional background as a developer, if just because it doesn’t seem like you have a lot of experience with system internals or with back-end development (nothing wrong with that, we need front end experts, too). Not that I have a whole lot myself, but I have enough to recognize some of the back end changes they’ve had to make to support Stage Manager. (I mention educational because I had to take some fairly low level classes for my BS, even a course on assembly language and one on graphics primitives, and it doesn’t seem like my school was unusually rigorous in its required classes for computer science.)