I conceptually agree that managing windows should be the task of... the window manager, but at the same time it's easier and lower friction to both develop inside of an app and get some user to test new features
Exactly. I agree that the OS/window manager is the “proper” layer in theory.
In practice, building it as an app makes experimentation much easier: cross-platform, lower friction to try, no need to replace your current WM, and we can iterate on workspace-specific features like terminals, browser panels, agents, Cmd+K search, worktrees, docking/tabs/splits, etc.