Clicking a logo just has it fill up your full viewport, it doesn't push an entry to the browser history or navigate to a new page.

They might want to add a zoom-in effect so that it's more obvious.

4) press Escape to exit image preview

This was done on a desktop computer. I have no idea what the experience is like on mobile.

Know to tap whitespace outside of the bounding box?

I presume, works like this on desktop. Only found this out after several muscle memory browser back attempts (mouse button)

Just tried this on desktop after reading your comment and can confirm it works. Didn't try it before because I didn't see an X anywhere on the screen.