Where does this advice come from? Most web servers would use a lot more resources if they didn't use userland threading, wouldn't they?

I think you're confusing userland threading (also known as green threads) with a regular thread pool implementation (that uses regular OS threads).

Because most web servers use OS threads. Nginx, Apache, litespeed, they all spin up `core count` threads but they can all work single threaded just fine as well.