I think both fpm and wsgi would fall under what OP called "proxying to application servers", despite using a different protocol. Yes it's technically "serving", but so is proxying and literally anything else a "server" does. Maybe "hosting" is a better term: both can serve any content, including dynamic, but apache can often host it as well (without an external server), which nginx can't do nearly as well.
What about the rest of that sentence? The way I read it (admittedly, the English is broken, so it isn't entirely clear what they meant) was that nginx is only good at "serving" static content, while the rest of what it does is proxying, not "serving". In that case, where a distinction is intentionally made between "serving" and "proxying", taking request for dynamic content and just shooting them off to an application server like uwsgi wouldn't be considered "serving".
Taking this logic to the extreme, socat is also very good at serving dynamic content:
$ socat TCP-LISTEN:80,fork,reuseaddr TCP:127.0.0.1:8080 # Make sure Gunicorn is running on port 8080 to handle incoming requests
