The scary thing about a supply chain attack is that Hezbollah aren't idiots. This is basically like buying a "burner phone" (that name will now have different connotations now).
In the movies, people buying burner phones go to a random corner store and buy a random phone off the shelf. That way, even if they're under surveillance, the cops / CIA / FBI can't pre-bug the phone because they don't know which corner store the person's going to go to, let alone which phone they'll pick off the shelf.
If you're an armed group in Israel's crosshairs, you're going to take similar precautions when buying thousands of pagers. The safe way to do it would be to slowly and unpredictably get a small sample of ones that are being sold to the general public. If this is true, it could mean that there are tens of thousands of pagers out there that contain explosives that were merely sold as "decoys" in order to try to make Hezbollah feel safe in buying them. In other words, there may be tens of thousands of explosives in pagers that weren't activated because they weren't in the hands of Hezbollah when Israel decided to hit the button.
If the device in my pocket started smoking like that, I'd throw it away pretty quickly. I suspect these were actual explosives, and there are almost certainly more of them out there.