We have a similar system in the Netherlands, only it doesn't use an app. When reporting a heart attack, the emergency response service will dispatch two ambulances and activate the system to summon volunteers that live or work nearby. I believe they get a robot call as well as sms text instructions. Half the volunteers get instructions to pick up a specific AED somewhere first, the other half is instructed to rush to the victim immediately.

I've had the misfortune to gather some anecdotal experience with this system in the past year. In both cases, 4 volunteers showed up very quickly, well before the first ambulance (which in both cases showed up within the allotted 15 minutes of the emergency call). One of the victims was my father in law, who is alive and well today, most likely thanks to the volunteers. Unfortunately, the other one -a friend- was not so lucky.