All of those things seem like they would be true.
But even on a very basic implementation you shouldn't have to hit the disk all that often unless you have an extremely large number of static files to serve. This is quite possibly the case for many sites, but I think for most sites, all of your static files could be cached with a minimal memory hit.
The reason it is advised to not load static files via Node.js is that nodejs is a single thread process, and static files will hold up this thread. Nginx on the other hand, is multithreaded, and can continue serving other assets and requests while the statics are being served.
-Node has more cycles to compute things?
-The server node's on has less contention to deal with (disk, network)?
-Putting statics on another server allows the browser more simultaneous connections when loading the page, so it loads faster by default?
"Don't use Node.js for static assets" is an interesting observation but I'd like to know exactly why.