How did you figure this out?
By actively using and tweaking the presentation mode for half a year. Later, I also inspected Logseq’s source code.
Did you find any other hidden features?
Pretty much every function of reveal.js works except Speaker View, Print and Code Animations. Plugins that do not rely on external APIs and render in blocks also render inside the slides with differing results. For example, Logseq Furigana, logseq-mermaid-plugin, haydenull’s Exalidraw, Block Calendar, etc. render, albeit some require writing some custom css for better rendering or a bit of thinking outside the box. Plugins like Markmap, Diagrams as Code, etc. do not work.
See also Patching up the Presentation Mode