I came here wondering the same thing. Based on the comments above, and what I know of the two editors, and the fact that there were some improvements to minihtml and suddenly today the sublime text changelog popped in HTML format, I would guess that minihtml is a light html renderer that can be used by plugins inside of sublime text. Atom already has this ability by design, so it can do things like draw a usage graph in a side panel, while previously sublime text could only have text-based panels produced by plugins. If I am correct, minihtml should allow plugins to display more complex user interfaces and data through the a minihtml pane.
