I actually consider the pager attack to be legal. There's obviously criticism of it, but I'm fairly sure you're allowed to do this kind of thing by laws of war. Obviously this creates a huge problem for pretty much everyone though, since we can imagine that our ordinary consumer products from all sorts countries could similarly explode if we ended up at war with the manufacturers.
I don't know if it's "legal" or not and by who's laws, but it certainly seems like terrorism to me (i.e. intentionally creating a state of terror). I think if Lebanon found a clever way to assassinate the top 45 military commanders in Israel the same people who are defending this wouldn't be calling it a "Legal act of war".
That's not really a good description of terrorism. Terrorism is going after non-military targets, or at least indiscriminate targeting, for the express purpose of causing terror.
If an enemy tank platoon is rolling down the street, the operator of an antitank missile certainly knows that blowing up the lead tank and killing the crew in front of their compatriots is going to instill terror in the rest of the tank platoon. Taking that action anyway is correctly described as an act that intentionally instills terror, but that's not an act of terrorism. War, regardless of if it's waged lawfully, is often terrifying.
The way to successfully argue that Israel's pager attack was an act of terror is to show indiscriminate targeting - not merely highlight how terrifying it is to have a bunch of high level officers killed at once. However, investing a lot in the latest information gathering technology sound like the opposite of indiscriminate targeting.
I obviously can't speak for how the public writ large would react to our hypothetical. But I can at least speak for myself that if Hezbollah somehow, say, flew a bunch of drones onto IDF bases and killed officers, then that would be an act of war but not an act of terrorism no matter how terrified it might make Israelis feel.