Lithium-ion battery fires are dangerous, but small batteries alone don't usually cause this much damage.
Both Wiens and MG said a supply-chain attack in which a remote-triggered explosive was surreptitiously placed into the pagers before they were distributed is more likely. There is precedent for this: in 1996, Israel put a bomb inside of a cell phone and used it to kill Yahya Ayyash, who was then a bomb maker for Hamas.
What I find most concerning is that a large number of people/journalists considered it plausible to blow up a (edit: stock standard) pager remotely via some hack or zero day.
It's plausible. If the pager had some kind of battery that could explode if charged, discharged, or shorted in some way, and the controller could be compromised, then it'd be possible to make it explode.
Remember, Israel worked on the Stuxnet attack that destroyed Iranian uranium centrifuges by infecting their controllers, making them go too fast, and destroying themselves.