When Jake first brought Billy Robinson to America from Japan (the story of him living in Japan is quite sad after his pro wrestling career, but that's an aside), Robinson needed an assistant and essentially Uke because of Robinson's physical limitations. His first set of camps in the US had at most a dozen people attend. Most of these guys backgrounds were either BJJ, varying levels of wrestling experience (folkstyle, freestyle or greco), CSW and traditional martial arts experience.
Kressin started going initially after one of his BJJ students went to a camp and noticed an improvement in his positioning and riding, and was curious to see for himself. Kressin trained under Robinson as much as he could over the last 6 years or so.
The original idea of becoming a full coach under Robinson was based around the 10,000 hour principle, sadly Robinson passed away earlier this year as most people know. It's good that his lineage is continuing on though.