If they were on a 15 minute period the nannies could be quite a distance from any one particular apartment. We would have to know how frequent the listening was done and the spread of the apartments to be sure.
If they were on a 15 minute checking period then that's twice as frequent as the McCann's chosen frequency, so that's another difference. I assume the nannies had charge of different areas so it's the size of those areas which matters, not the spread of the apartments as a whole. Also the nannies were around. They weren't in a bar or restaurant between checks, they were outside walking the paths and able to see and hear anything suspicious. Here's an account suggesting 15 minute intervals with the MW system, along with a normal parent's misgivings about the system;
We went to Sardinia with Mark Warner last year and children were unwelcome at dinner. You were supposed to use the baby listening service, which consisted of leaving the child alone in the room and every 15 minutes a nanny would physically go and stand outside the door to listen. If the child was crying she'd walk over to the restaurant to fetch you. A lot of parents used this but I thought it was unsafe and mean to let a child cry in a strange hotel room all alone. Also, my son could climb out of the cot and knew how to open the door to the balcony, so I thought he could easily have met with accident in 15 minutes. Why don't you ring up the hotel and ask exactly what they offer and then you can decide if it would suit your child?
https://www.mumsnet.com/Talk/general_advice_tips/2633-baby-listening