That figure comes from the ship's documents. Yet there are estimates that suggest the actual explosion is only equivalent to 1000-1500t[0], or 700-1000t[1].
If the actual amount of nitrate was lower that the stated figure this strongly suggests some form of smuggling was involved, and someone was siphoning off the material. This would immediately explain the enormous 'negligence' that let this incident happen in the first place.
[0] https://www.bbc.com/news/world-middle-east-53668493
[1] https://www.occrp.org/en/investigations/a-hidden-tycoon-afri...