Plain HTML and one CSS file (read: one, not five, not ten CSS files) minus the JavaScript bloat would also load insanely fast. It doesn't have to be AMP.
Try the basic HTML verson of Gmail -- it loads about 5-10X faster than the AJAX version.
That's actually completely untrue. Every AMP component is a web component and requires JavaScript in order to run at all. And many have their own .js file powering it. You literally can't use an <img> tag on an AMP page. You have to use <amp-img>, which has a JS dependency.
OP is right, the primary reason AMP is fast is because Google is preloading the page in the background. AMP pages themselves aren't all that static, or really all that bandwidth efficient.