Earlier quoted context omitted.
The specs for the watch specifically rule out heart attack detection, by the way. > Apple Watch cannot detect heart attacks. If you ever experience chest pain, pressure, tightness, or what you think is a heart attack, call emergency services immediately.
"What we list for legal purposes" and "what the device actually does" might not be the same thing. I imagine saying "my device detects heart attacks" gets the FDA involved and makes everything ten times more expensive...
Outside possibility is he felt bad, saw a heart rhythm notification, and manually triggered emergency sos by holding the power button. But a fall alert seems much more likely.