I've just been poking around the international Conventions, and it does look to me like Israel signed up to banning "the use of booby-traps and other devices in the form of apparently harmless portable objects".
https://disarmament.unoda.org/the-convention-on-certain-conventional-weapons/high-contracting-parties-and-signatories-ccw/
So I suspect that when people are calling the pager explosions "a war crime", they're referring to that agreement - not the variable of who was targeted.