He forgot to mention that iPad sales are in the gutter. Technically, even if what he says is true, they could probably have gotten away with 3 virtual workspaces instead of 4 on older iPads. 2018+ iPads are plenty fast.
Personally, the more I think about it, the less I like the idea of not utilizing a big chunk of my screen real estate just to switch between apps. Most people I know don’t swap between groups of apps, either, which stage manager seems to have been optimized for… like virtual desktops.