> ammonium nitrate which only becomes an explosive when mixed with other chemicals

This is incorrect, ammonium nitrate is intrinsically high-explosive. It is mixed with other ingredients to improve performance and/or sensitize it but this is not required. There are several famous examples of stored ammonium nitrate exploding e.g. https://en.wikipedia.org/wiki/2020_Beirut_explosion

The confusion likely stems from Ammonium Nitrate (uncontaminated) generally only being explosive when hit with a very high speed shockwave (aka a booster charge + detonator in typical mining/blasting applications), or when sensitized with other chemicals (like with tannerite).

Most of those other blasts, including Beirut, involved lots of contamination of the AN + large fires + very large quantities of ammonium nitrate.

In truckload or less quantities, and when not contaminated with specific substances, ammonium nitrate is pretty much impossible to detonate without a sizable quantity of some other high explosive.

You’re of course technically correct, but in practice a truck of ammonium nitrate is basically inert on its own.

If you’re going to assume they have access to significant explosives to actually detonate pure ammonium nitrate, then you also need to consider flour and other ‘safe’ compounds due to the potential for even larger fuel air explosions.