From Brian Norris:

https://support.google.com/chromebook/answer/7225252?hl=en

It sorta suggests that we don't support USB:

"You can print from your Chromebook using most printers that connect to Wi-Fi or a wired network.
Note: If your printer doesn’t connect to the internet, you can’t set it up with your Chromebook"

But then it does have a note later that suggests USB can work:

"You can also connect your printer to your Chromebook with a USB cable, then print. If so, the printer doesn’t need to be connected to Wi-Fi after it’s set up."

Seems like it could use some improvement.