No, it can blow up all on its own. The oxygen in the NO3 is perfectly adequate to decompose the NH4. Not likely if it’s completely pure and dry. But 6 years in a dank ships hold can easily produce moisture, and as fertilizer, it’s probably not lab-grade purity to begin with. The decomposition of the NH4 is super exothermic and at least partially produces NO2, quite flammable. Once the reaction gets going, it runs away.
The materials spent the last 6 years sitting in bags in a warehouse along the waterfront. The materials reportedly were produced in Australia as an explosive ingredient, not a fertilizer, and then ended up in the warehouse when the ship they were on came into the port with mechanical problems.
Supposedly the materials were on the way from the country of Georgia to Mozambique.