What do you mean? More ambulances would mean you could be nearer to the patients (imagine an ambulance on every street corner) so the net journey time would be less. You can also administer care from the ambulance (e.g. narcan, cpr, stopping bleeding etc.) which can save lives. A huge number of ambulance journeys aren't time critical either (e.g. a broken leg) but not having an ambulance means getting a taxi or other transport which exacerbates problems.
Probably better to have more ambulances overall, and maybe a few ultra fast ones (e.g. helicopters) for time critical situations.
It is obvious to me that you're correct though. Overall, the throughput matters. There are only so many people you can pack into a single F train and it won't help much if the train could go 60 mph all the way if the only scheduled trains arrived every two hours.
I think they're trying to talk about latency versus throughout. Latency matters more than throughput for an ambulance if you assume that only a small portion of the population will have a heart attack or needs emergency medical attention at any given time.
Probably better to have more ambulances overall, and maybe a few ultra fast ones (e.g. helicopters) for time critical situations.