I am mildly drunk so this might sound retarded, but I don't think Jackson was just the social media guy for SuperMega. He is specifically described as having fiduciary duties within the company, with Matt and Ryan as the principal. Through these legal terms its entirely possible that he was the director of supermega or finances guy of the company, which could explain a bit.
Timeline could have been - They hire a tax guy for 2020 tax season, who notices the company's credit card has a good amount of charges, but nothing egregious. Tells Jackson, who blows it off as business expenses for skits (about 10 grand). Explains why he only spent $120 on it in January 2021. After the tax guy buys that story and doesn't mention it to the owners (Matt and Ryan), he decides he can go full send and just use the card as his own, which is how he racks up over 40 grand in charges in a year. Now that its an insane amount of money the tax guy goes over his head straight to Matt and Ryan, who promptly fire him.
Matt and Ryan are just the funny comedy owners/actors of the company, so Jackson being their finances guy would make sense to me. I mean its not a lot of work at all to run a couple social media pages like SuperMega's.