Conjecture: Hugo and friends generate static file sites can touch on high signal for search engines: fast load times, meaningful cross-linking, responsive templates, minimal (if any) cross-domain loads, and new in 2020: minimal 'cumulative layout shift' and 'largest contentful paint'.

Not to say that you couldn't construct some monstrosity of a site with Hugo, but you're more likely to do that with a mess of half-maintained Wordpress plugins.

Is the HTML that Hugo generates semantic and does it contain the usual metadata that search engines use?

Hugo’s HTML generation is very lightweight and basically outputs whatever your HTML theme template tells it to— so with a bit of work your site can be very SEO-friendly.

This is up to the template files.