I wouldn't say Jock is a "Mondo artist" nor would any artist doing a non Mondo print be considered "side work". These guys are artists, and Jock is actually considered a comic book artist. When I met Jock at Chicago Comic Con I felt weird being the only guy who didn't have a comic book for him to sign. You know, there were guys in line who were super geeked about the work he did on one of their favorite books and I'm standing there with a copy of The Raid in my hands. He was still super proud of his steelbooks but even the Con guide listed him as a "comic book artist".
So yeah, it does get confusing. A lot of artists who do work for Mondo also do work for other art galleries, comic book studios, movie & television studios and even advertising. Mark Englert designed a Jurassic World edition of Barbasol Shaving Cream.
Rich Kelly did the Winter Soldier poster for Mondo but also did Dr. Strangelove for Gallery1988 which was released as a steelbook.
Jock's The Raid was Mondo but The Raid 2 was commissioned by the studio.
A lot of great artists out there getting their work on steelbooks and blu-rays but as it is there are only 4 steelbooks with Mondo art that aren't included in the X line; The Raid UK & DE, Ender's Game UK and True Detective.