Part of plausible deniability is that it has to be plausible. There has been no plausible argument presented that Israel did not do the pager and walkie talkie attack. For that matter, there hasn't even been a denial about it.
In war there are still rules of engagement and expectations about things like “child soldiers” and “civilian casualties” and “collective punishments” etc….
But also, how much to those rules actually stop people?
Which rules are worth breaking if they prevent open war and millions of deaths?
No idea. Some deep philosophising and rationalisations around all of it is required regardless of your stance