I don't generally defend Jobs (he was apparently a world-class asshole in general), but the implication in the story seems to be that part of the reason he acted the way he did is because he knew other people in the company—possibly including the two people he had promised 50% to—were potentially listening to the conversation (remember, it was in a cubicle).

In other words, Steve's behavior was at least in part a performance intended to to demonstrate that he was holding up his end of the deal he had made with the developers.

[Of course, the manner he did it in was obviously still pretty obnoxious—he could easily have been much more gracious while still insisting on that number. I'm just saying there did seem to be a concrete reason underlying his actions, it wasn't just him being a jerk... ^^; ]