The batteries are the only major energy storage device there to breach.
Clearly Israel has found a software vulnerability that lets them overload some otherwise minimally used processor and overheat the batteries. Above ~140f lithium ion cells go into thermal runaway.
I was discounting the possibility that Mossad was in control of global supplychains with sufficient intensity and recklessness to hide semtex and a detonator in an entire brand of consumer electronics, in its urgency to provide retroactive justification for every antisemitic conspiracy theory out there and ignite war with the entire Middle East.
The only thing in a pager that SHOULD BE THERE with enough energy to 'explode' on demand, is a lithium ion battery. Which is evidently, given further reports, not what happened here.
It seems like they actually installed small bombs in the devices.