Meaning they said "i'll work another 3 months, but I found a new job" and their current employment said "nah, today's your last day". I've seen it happen first hand.
I gave something like 6 weeks notice at my last job and by mutual agreement spent most of that time doing presentations and updating docs to sunset my bus factor. The mistake isn't on the part of the worker who has the courtesy to minimise the disruption of their departure, it's the employer who poisons that well.
Of course, power dynamics being what they are, it's the worker who pays the price.
Returning to add: the benefit to employers of acting like their (ex-)employees are human is that we finished off with a very friendly "if you're interested in coming back, the door is always open", and indeed during my time there I had more than one ex-colleague who became a colleague again. Often at a much higher level and salary bracket, tech promo being equally broken everywhere.
Personally I find "bus factor" to be one of the less egregious pieces of corporate jargon. It's a problem that mainly hurts the rank and file and is utterly irrelevant/invisible to management until it becomes very visible and we inevitably take the blame and eat the consequences. Learning to speak their language to communicate a very real problem is a key deliverable.
I do feel a bit ashamed of unironically using "sunset", although in my case it was more in the sense of "ride off into the ~". It was my last corp job (hopefully) ever.
