Ah, thank you. I didn't mention it, but I also noticed that a few of my sims had gotten jobs while I wasn't playing them, and figured that should probably also not happen while progression is turned off.
In fact, this can happen, wether story progression is on or off.
Whenever one of your active characters starts a new career, the game checks if there is a boss and employees already assigned for this career. If not, it will randomly assign some. This will happen, no matter if story progression is active or not.