A little of the fog of war perhaps.
It may more likely have been sodium nitrate (as other reports indicated: https://www.wsj.com/articles/lebanon-struck-by-blast-at-beirut-port-11596556605 ).
That is rocket propellant, and Hizbollah is in the business of building their own - at the direction of their bosses in the Iranian Revolutionary Guards.
Unmixed ammonium nitrate is not as explosive -its fertilizer. It can be mixed with diesel fuel oil to make an improvised explosive (ANFO), but Sodium Nitrate is more of a military grade substance. It was a Hizbollah facility, and there were a series of secondary detonation in the fire, before the big one went off - possibly other munitions or components.
Fireworks just sounds like a BS cover story, as does ammonium nitrate - Hizbollah is trying to avoid blame for having a mountain of high explosives downtown, and mishandling them.