I think they'd have much more of a defence to this than to anything in the current South Africa-led case. I mean, these are (according to reports) pagers issued by Hezbollah to persons they've authorised to have them; just that almost certainly makes this a discriminate attack against Hezbollah, irrespective that they couldn't guarantee who was actually holding the device or near the device.
"Indiscriminate" in this hasn't ever really been defined, but it would probably only cover knowingly using indiscriminate / disproportionate means to attack when other means were available; eg: using a 2000lb bomb that flattens a block rather than a precision guided munition to kill one person, or firing artillery shells into a crowded residential area when PGMs are also available and could be used instead. TBF I don't even think "indiscriminate" would cover an attack when unguided weapons (like Hamas / PIJ rockets) are the only weapons at hand.
Now if they'd blown up every pager in Lebanon, that would be indiscriminate.