A LiPo will burn, aggressively and hot, but they don't explode.
To get a LiPo to explode you'd need to both puncture/rupture it and somehow contain the escaping gasses long enough to build up pressure.
No, I'm as convinced as I can be that this was a supply-chain attack, and used a purpose-built "addition" the pagers in the form of an explosively formed penetrator.
Given that an EFP is usually concave, I'll even go so far as to say I bet it was disguised as part of the speaker assembly.