The main thing the spinner waits on is the .epub file to be downloaded. That file is 4.77 MB, which is appreciable for anyone without a fast connection. Most of that weight is images (99% after decompression). Unlike a normal webpage (or a PDF), rendering doesn't appear to start until all the assets, the whole ePub, has been downloaded.

This segues into a point of difference I thought the article would mention, but didn't: performance.

A PDF can be optimized so the pages are substantially independent of each other, which makes rendering pages progressive, random-access, and highly parallelizable.