Pulling a resignation notice shows that relationship to have broken down, probably irrevocably, I agree. But, the employer initially broke the relationship, by not keeping their promise regarding the bonus.
The employer (perhaps here the boss's boss) knows they have treated the employee unfairly to an extent that could become legal, which would have an associated cost, and also could impact upon the other employees' motivation (and how employees evaluate the promise of future bonuses).
If the employee is essential and their replacement cost is high, then there's an significant additional cost to firing them fast.
The employer might have to weigh up the costs, and could reason that it's cheaper or otherwise better for the business to pay the bonus.
Or maybe the employee misunderstood the conditions of receiving the bonus. We can't really judge the situation after hearing just one side.
He asked for clear conditions and management agreed to provide them, but never did. They were playing him. The comments from the older employees suggest this wasn't the first time they used this tactic.
But I tend to agree with the comment suggesting it was probably to avoid legal action.
It's definitely a nuclear option, which most often should be responded to with, "okay, do what you have to do".
It is certainly not true that it is never viable to pull the resignation notice as an ultimatum. Simply depends on who has the leverage.