This is true. As others have pointed out, all PO Boxes can be appended to a 5 digit zip to get to the box (e.g. ZIP-BOX#) No city or state should be needed (zip covers that), nor name (not needed for mail in general, but might cause other issues).

The US Postal System actually uses a full 11 digit number to identify a "Delivery Point". The "+4" after a zip code is basically a subroute within that zip code, but there are two numbers after that you don't really see that will identify the full delivery point, which is basically a mailbox. The USPS tracks a number of things at this delivery point level, including whether a secondary address is needed, if they are currently delivering to that address, or if service is temporarily suspended (potentially due to a natural disaster or road outage in the case of a more remote rural address).

I'd really love to see if just a full 11 digit number is enough to get mail delivered to my house. I guess I could try, as it's easier than you'd think to find out your code