Sorry to say but not great opsec by Hezbollah. I get not opening all of them but they should have opened some at random just to check for the possibility of electronic hardware bugs. Unless it was packed into the lithium cells themselves which might be possible. Heck for all we know they found a way to mix explosive compounds into the lithium itself in a way not visible by physical inspection and only by chemical analysis and perhaps subjecting them to a higher than normal voltage or something triggers the reaction.

Edit I really am going to lean on the idea of either some sort of contaminant introduced into the lithium cells designed to make them more dangerous and likely to explode rather than just burn or some sort of explosive compound mixed in with them that required the battery be overloaded for it to go off.