 If the simple predication swim(Ewan)(e) in 59(b) could be understood as “e is an event in which Ewan swims”, then 59(b) could describe an event where Ewan swam for just five minutes, but a lot of other things went on as well in that event: He rode his bike, his sister slept, his mother harvested shallots, his father irrigated fields, and taken together, those activities took a total of 10 hours. 59(a) doesn’t describe events of this kind, hence 59(b) couldn’t be a formalization of 59(a). The standard way of understanding 59(b) is as saying that there was a swim by Ewan that took 10 hours.