Just having watched the video, the session setup, close, and launch seem tied to the idea of a single window per app. Is that right? What kinds of workflows is this explicitly meant vs. not meant to support?

The way a user switches window sets with backtick and app windows within them with tab is inverted from the usual major and minor shortcuts. Normally you switch apps with tab and windows within them with backtick, or in the case of stage manager, you switch window sets with tab and windows within them with backtick. As a default I think the keys make less sense than they could.