The story does help to fill in some of Scotty's background and his experience on freighter ships but it fails completely to justify his imposition as a chief engineer. Engineering skill justifies a job as an engineer - your chief engineer says fix the ship and you do it - but becoming a senior officer requires leadership skill and good judgment. Scotty, as portrayed lacks pretty much all the skills needed to be in charge.
What's more, the oddity of Keenser being a senior officer is underlined yet again. He's a lieutenant and has spent years as a chief engineer on the Delta Vega outpost. Scotty qualifies from the academy, presumably as a lieutenant in recognition of his years as an engineer on freighters, commits an illegal act of cruelty on a living being, and is sent to Delta Vega as a punishment for 6 months, where he treats his senior officer as a skivvy.
Keenser should have been a petty officer. His treatment in the movies and comics is an ongoing insult to his rank!