I investigated further and realized we’ve since incorporated a slightly more advanced heuristic. In addition to checking if the cell’s value is directly an SVG element (or a Canvas element), we test if it contains a single SVG element that covers more than 50% of the cell’s area.
The reason the Vega-Lite API notebook gets the Download PNG option, then, is that the charts are full-width! Whereas in your notebook, the charts are only 30% of the width (300 / 954) which fails to cross the threshold. You can get the Download SVG and Download PNG options to appear in your notebook if you make the window narrower!
This is again because of the wrapper DIV element that Vega-Lite applies to the SVG. DIV elements are display: block, which means that the cell appears to be full-width even though the contained chart is only 300px wide.