Yes, it counts.
nginx is written in C and handles all what shall web server
handle for your static files.
Apart from speed, you will also get lower load on your Python code.
Regarding caching - if you like,
nginx can serve for your
app also as a cache, see NGINX
doc. But if the request is to be processed by Flask, there is nothing
nginx would speed up apart of offloading your app by e.g.
serving static files.
The extra effort for configuring
nginx for static files
might not be necessary while developing an app, but for production use I
would recommend it.