Jet lagged 2:30am thought:
"Vertical slice" has always been a flawed concept, and the clue was in the name the whole time: you can't have a slice until you've baked the whole damn cake.
Cannot be played by players meaningfully but game dev can use it to confirm the overall plans work/tweak them and from there out it’s just painting between the lines
maybe…. the full game layout with only placeholders for everything, including menus and levels and hints of content. the point is that you can complete it, not that it is any good?