Thanks for making those points and for defending your position in the face of a challenge.
I am not suggesting that NH4NO3 is easy to detonate, though presumably it did not require a nuclear device in, say, Oklahoma City or anywhere else NH4NO3 has been used as an explosive.
To be honest, my first thought when I think of the possibility of a highly targeted mystery strike would be a hypersonic kinetic bombardment. I don't see the need to either invoke or explain the use of an unpredictable large-footprint weapon - right on your own doorstep - one that in the context of symmetrical retaliation of any kind, could wipe out a small nation as initial perpetrator.
It brings up an issue of testable/untestable weapons vs. those that really should be tested before use. If an kinetic bombardment gets effed up on your own doorstep, the ramifications are not too bad. So you may ask 'why does Israel have nukes?' It would be a good question, probably, and presumably the answer would be to use as a last resort against a respectably distant 'enemy' such as Iran. Or as you probably point out, the yield may be unusually low for a nuke. But to ensure that yield really is reliably low, would likely require some fairly persistent testing.
Beyond that, its an issue of evidence. Speculation is easy (and fun).