False positives are not really an issue. Last I heard 60% of chest pain cases ERs see are false positives (constipation or something like that doesn't require ER care). However it is sometimes the only symptom you get of real medical emergencies so you go to the ER anyway.
There are also a number of people who die of heart attacks with no symptoms. If Apple can get even 1% of them to the ER that would be a huge win. If the false positive rate is 80% that is still enough real positives that ERs will be get used to telling people "This time it is nothing, but it is good you came in anyway because sometimes this is all the warning you get".
The real worry is false negatives - someone who has chest pain and decides not to go because the watch says all is okay. These people will die when a hospital could save them.
There are also a number of people who die of heart attacks with no symptoms. If Apple can get even 1% of them to the ER that would be a huge win. If the false positive rate is 80% that is still enough real positives that ERs will be get used to telling people "This time it is nothing, but it is good you came in anyway because sometimes this is all the warning you get".
The real worry is false negatives - someone who has chest pain and decides not to go because the watch says all is okay. These people will die when a hospital could save them.