It has probably been easier in the Good Old Days to become boarded in most fields than it is today. A long time ago, in the 60s, Derm wasn't even a Residency, but a Fellowship subspec to Int Med, and waivers for foreign trained Derms was perhaps possible, but I don't actually know that for a fact.
However, it is in fact still possible to get licensed today, without going through a US residency. The pathway is to be hired at a tenured academic level by a medical school, which would allow someone to get licensed.
There's actually quite a few of those around, including, interestingly enough, several Program Directors. I know of the Swedish vice chair and PD of thoracic & cardio surgery at the preeminent US heart center, Cleveland Clinic, and the UK director of surgical pathology & Program Director at Brigham & Women's, as well as several Harvard attendings.
However, these individuals cannot claim to be board certified, because they aren't. But they have a medical license, and can as such work as physicians, without a board certification.
As per your specific question, however, I don't offhand know any Derms who haven't been board certified. But bear in mind, that Derm hasn't always been as competitive as it is today. Conversely, you see quite a lot of FMG's in Psych today, which was in fact one of the über-competitive residencies in the 1960s. So shifts in competitiveness is the most likely explanation for your observation.