FWIW, the debate between you and u/FSYigg is a little weird because you're both half right and half wrong.

You don't need an AED to detect AF. You just need an ECG. AF is diagnosed from irregularity and atrial activity as P waves, which the very basic-level ECG capabilities of the Apple watch are very capable of doing. A clinician can also fairly reliably detect the signs of AF with a stethoscope, although formal diagnosis would need to be confirmed by an ECG. More widely, cardiac problems generally need a 12 lead ECg, which the apple watch does not do. An apple watch can definitely detect AF. But it shouldn't be relied upon for formal diagnosis.

In other words, it's not a "medical device" in the sense that no doctor is likely to begin treatment for AF based on your own findings from a watch, but they would take a watch ECG seriously and reliably enough as a sign that further investigations are needed.