@John *********
this is what I always went by, and every experience I personally had follows it. That's where my confusion comes from.
According to the UN Traffic Act of 1949 and the Thai Traffic Act of 1979, an IDP is not required if you are a tourist/visitor in Thailand as long as your license is in English, has a photo, and your country is a contracting state of the 1949 treaty, which most are.
Driving in Thailand - U.S. Embassy Bangkok
th.usembassy.gov › u-s-citizen-services