I can't think of any reason why they would be. SEO things to do on your own site are all about content, markup - nothing about a tiny site is a problem here. In fact, speed of website is (was?) a important SEO factor - so by definition tiny websites start with a massive plus.

Why are they bad for SEO? The author seemed to be talking more about CSS and JS rather than content - does not using bootstrap or ReactJS mark you down on search results?

IMHO, SEO is just shorthand for SnakEOil.

If gaming search engines is your Brad-and-butter, is your idea really that novel?

It's not about gaming them anymore.

It's about structuring pages and content in such a way that Google et al believe works best for the reader.

Due to Google-side analytics, it's also about bounce-rates. This leads to both more in lengthy pages with depth, as well as lengthy pages of bullshit.

Why would they be? They are extremely fast, and over 90% of the page is the main content. There is very little noise, so Google can parse them easily.