"Consider using the "304-Not Modified" response code on large websites with lots of pages that don't change very often."
This is the only thing that, from an SEO perspective, I'd challenge. I can't even begin to see where the supposed benefit of this would be.
For context, I've only come across a HTTP 304 status once in 9 years of SEO and crawling websites on a daily basis. I've no first hand experience of them being deployed in this way at scale on a live website and so haven't seen any server log analysis that demonstrates the efficacy of their usage etc. But it's an interesting idea nonetheless.
With dynamically generated content you’re more likely to just see 200s, but I think Nginx sets Etags automatically on static content so it’s common to see 304s there.
I’m pretty surprised you haven’t seen it often, but I’d guess it’s more to do with whatever crawlers you’re using (they’d need to be caching content and headers), rather than the scarcity of the status code.