He may just be a tad force sensitive and know a few tricks. That's it. Certainly doesn't know enough to become full dark side.
He does seem to know some genuine Jedi aphorisms though, basically verbatim. The interesting part is that they came out not when he was leaning into the con that he was a Jedi, but much later when he seemed to genuinely be trying to give life advice. That he seemed to drastically misrepresent the intent of those words only serves to speak to his own personal outlook.
The most likely explanation is that he was at least partly trained, either at the temple or as a "bokken" Jedi. Assuming Jod is about the same age as Jude Law, he'd have been around about his mid-20s during the Clone Wars, so a little young to have still been a padawan, meaning he was either a full Knight that survived Order 66, or he could simply be a wash-out that left the Order years prior.
I'm curious how Rennod got the mint ship in the first place. Plus, why was it crashed/hidden in the forest? What happened to Rennod?
Good question considering by Rennod's time, At Attin was already a legendary lost planet, so it's not like he could have hijacked it en-route since the shipments hand long since been halted.
Given that it looks to have been in very good condition before he clad it in armour, it may have been brand-new, straight off the production line. Which means it was either in some rich person's private collection (and they presumably didn't know what they had), or salvaged from the hangar holds of some giant bulk freighter left adrift for centuries.
Given the extensive modifications, it seems like Rennod had the Cinder for quite some time before finally tracking down At Achrann's location, so he may not have realised what he had initially . . . or there was some other train of breadcrumbs to follow that led him there; maybe even the very same symbol that Kh'ymm to interpret the coordinates?
Given that when the kids found SM-33 his hand was holding what is presumably the main power breaker in the "off" position; I'm guessing Rennod had SM-33 cut the power mid-flight after in concubine stabbed him to make sure neither of them got the loot.