I think he meant if he leaves, his coworkers would have to take on his tasks. That’s why I like sprint based development - if my boss wants me to do something unplanned, he has to remove something planned first.
Yeah, after almost a decade of “agile” I’ve never seen this to be the case other than when I was super green. Usually, everything’s high priority and it needs to be delivered by day XYZ/ASAP regardless of sprint alignment or milestones.
My boss and I plan tasks at the start of each sprint, and once the tasks are chosen the only way to add a more is to remove something from the sprint. Asking more output from me than what was agreed upon would be similar to me asking my boss for more money. In both cases it’s perfectly reasonable to refuse the request (unless I underperform or am underpaid), and if either side is not satisfied, part ways.