Not to diminish your point, but to add to the discussion.
I would point out that landmines are also indiscriminate and allowed within warfare. If anything mines are slightly more indiscriminate due to you not needing to have accepted a device.
Although I think morally people are against the use of mines, we've seen widespread use of them in Ukraine. It would be good to see a global ban on these type of methods.
I think you nailed the analogy with mines and the distinction between legal and moral.
Putting morality aside and focusing only on language and semantica:
I agree that this action may be better described as military action with an associated risk of harm to civilians (like many military actions do) rather than a pure action against civilians with the purpose of terrorizing your enemy civilian population.
I don't think it's common to use the word "terrorism" to indicate acts whose purpose is to induce fear in your military opponent.
Perhaps "psychological warfare" or "intimidation tactics".
That doesn't forbid you from qualifying that act of aggression as exacting an unnecessarily high toll on innocent bystanders. You don't have to invoke the word "terrorism" if all you want to say is that an act is immoral.
> landmines are also indiscriminate and allowed within warfare
That is an interesting angle.
I do see that there is a clear distinction that makes the pager/radio explosives worse ethically. Landmines are generally laid to prevent people from encroaching on a guarded area. Minefields can be labeled, which suggests that the idea of the presence of a mine actually helps them be effective in preventing encroachment.
In the pager/radio case, the explosives were distributed to individuals (both devices are usually worn on the body) with zero indication that there was danger. There is no 'protection' being done here, just murder, and loosely targeted. When the attacker is ready, they detonate 4000 devices without knowledge of the environment around it, meaning they are willing to deal with innocent people being killed.
It's a really evil tactic.