SSG gives you the best SEO because web crawlers can understand the structure of a static page with pre-populated content better than a dynamic site that relies on Javascript. The pages also load much quicker and score better on other metrics that Google uses. I would go so far as to say that SSG is a must for major publications or anyone serious about SEO. This sort of data-driven SSG is exactly what you are thinking of but it happens at build time so that the build output is static content that can be put behind a CDN.
Absolute nonsense. You don't need a static site generator to serve up an HTML page instead of rendering with JS. People have been doing dynamic pages since before JS even existed.
The difference in speed between running a simple script and reading a file is not large enough to noticeably affect SEO.
Most of these sites are probably just using PHP, not any static site generator.
