“Expected” is not enough. These bombs didn’t go off in active war zone. They went off in public in Lebanon, and maimed and killed civilians.
The principle of proportionality is explicitly about expectations, i.e. expected military advantage vs expected collateral damage.
You seem to be holding Israel to an impossible standard of guaranteeing zero collateral damage, which IHL does not require because no military is capable of that.
Hezbollah was actively launching thousands of missiles at Israel when these pagers blew up. They stopped launching missiles at Israel en masse soon after these pagers blew up. What a odd coincidence.
I found this thesis from some guy doing a master in international operation law at the Swedish defence college, https://fhs.diva-portal.org/smash/get/diva2:1974147/FULLTEXT...
and I interpret his analysis as that it was targeted enough to be legal.