No he had an ERFS document, which is in effect a visa invitation letter that you bring to the embassy or consulate to document the receiving/inviting organisation. These are quite common to apply for visas for many countries. I would imagine certain nationalities would even need them for entering the US. I have been issued more visa invitation letters than I can possibly remember.

The issue is that many of us EU and US passport holders have forgotten what it actually means when you are required to go through a full visa process. As we had the benefit prepandemic of very rarely having to do it. The ERFS process was not particularly badly described or cumbersome at the point where you could start entering as self organised tours.

The surprising point was that it was there for a country that we fortunate EU and US passport holders used to be able to just waltz into, and people got caught in not having done their preparations. And United got caught in not checking the documents properly.

No wonder ANA and JAL does a full document check for connecting passengers before letting them board the plane to Japan.