nah... there was a vector of hydrogen gas and lead acid batteries, an episode of Mr Robot featured this, in a mostly closed environment, and entailed haxing the ventilators to turn off so it reached oxidising ratio

that is a whole different kettle of fish and air that has enough H2 in it to go bang in any serious way is already too low in oxygen for humans to be in it

plus it's pretty hard to get lithium batteries to leak much hydrogen

no, these old school pagers had probably got special batteries with little globs of C4 explosive or semtex or something and a small, passive radio device embedded in them that waits for a specific coded signal on a specific set of radio frequencies to charge their capacitor and discharge into the C4 blob

most likely the devices had NiMH battery cells, which are quite bulky for the power capacity, as these are old school tech, and probably don't work with the new 3.6V lithium battery voltage but rather the old school 2.5-3V of 1.4V/cell NiMH