20 grams of C4 or similar plastic explosive would be more than adequate to produce the effects seen. CL-20 wouldn't be a good choice for this deployment: it's not particularly stable for rough handling even with a good phlegmatizing blend, and tends to decompose at the temps one would expect a pager to be exposed to (hot car, etc).

That's a fair point my knowledge of explosive chemicals only stems from conventional missile-based warheads and propellants. It's very possible a different explosive was used.

C4 would be easily detectable by spectrometers, including at the airports, wouldn’t it?

per a calculator on unsaferguard.org 20g of RDX

Injury/Fatality to Personnel Range (m) Fatal Distance 0.64 Lung Damage 1.02 Eardrum Rupture 2.63

So apparently, it must be less than 20 g of RDX, and/or a weaker explosive: if the fatal distance was within 0.64 meters, there would be significantly more reported deaths.