Clinicians report as little as zero success for patients in asystole (ie their heart has plain stopped). Some studies report a little higher, but it's also possible those rates are mistaken diagnosis, ie you see a flat-looking ECG, you treat anyway, maybe 1% of patients survive - but maybe that 1% were actually not really in asystole.
If you have a heart attack, both an AED and CPR are inappropriate, your heart is still working it's just that the blood isn't going where it's needed. You need immediate medical treatment. While an actual cardiac arrest (which would warrant an AED and CPR) is much more likely during a heart attack, it is not inevitable and these treatments are not prophylactic. Meanwhile your focus needs to be on alerting other people to your problem, so that they can get you medical treatment, and if you do go into cardiac arrest they can perform CPR or use an AED once that becomes appropriate.
Isn't that basically the no true Scotsman fallacy?
Sometimes, when our measurements differ from what we can explain with our understanding of the universe, that's because we didn't understand the universe properly. If you put the Sun in the middle, your model of the solar system is still weird nonsense with planets on their circular orbits needing to swing backwards or accelerate forwards for no reason - until you make the orbits ellipses instead and then it's almost like fucking clockwork.
But sometimes, you were just right, and the measurements were off a bit, and there was no real physical phenomenon, you were just bad at measuring.
So, maybe sometimes the human heart does actually stop, yet it can successfully be restarted and then carry on as normal, and the circumstances where this is true are just fairly uncommon and when people are peering very carefully at the results they never ran into one of these rare cases. I cannot rule that out.
This is a phrase that's easy to interpret in a dangerously wrong way. Here's a clarification:
If you have a heart attack and are still alive and talking, both an AED and CPR are inappropriate.*
If you have a heart attack and die, then both an AED and CPR are very appropriate and might bring you back.
* And even in this case the emergency response medics will put AED electrodes on your chest so it will be ready to use in case you crash.
If the patient has truly flatlined (asystolic) then shocking them won't bring them back. That only works in the movies.
AEDs are fucking magical.
They even talk to you to tell you how to use them.
If the need arises, get the AED and get going.