Especially CPR alone. Assuming a defibrillator wasn't used, there's a non-zero chance this wasn't actually a cardiac arrest, but rather a loss of consciousness

Especially CPR alone. Assuming a defibrillator wasn't used, there's a non-zero chance this wasn't actually a cardiac arrest, but rather a loss of consciousness and the bystander couldn't find a pulse that was actually present (finding a pulse in a high stress situation is not at all easy to do).