I mean the article itself says that the other improvement are much less significant than the overall 2.5 speedup of the amphtml page. Whether the user will have to do 1-2 more seconds is immaterial , when you have typical newspaper websites that take 10-30 seconds to load.And you don't implicate google's caches (which will at some point refuse to render you if your page is against their politics) in any step.
Regardless, i believe amp will fail equally to how html pages fail now: newspaper marketers will keep adding more and more stuff to their amphtml webpages (and amp makes it easier to add extra components), and no matter how fast it loads, they will become heavy as hell and your phone will again start to smoke. We just dont see that yet because it's early days - but newspaper sites were also bare html 20 years ago. AMP is an evil bandaid with significant sideeffects