Batteries aren't pressure vessels though. Pressure vessels are generally decently large; how are you going to get one with significant capacity inside something as small and lightweight as a pager? Just putting in some plain explosives makes a lot more sense.
It's a very minimal amount of pressure it can withstand, is the point. Certainly nowhere close to lethal explosive pressure. It's not a pressure vessel in the sense of the kind of pressure vessel it takes to make an effective bomb.
