Furthermore, this wasn't ANFO, just ammonium nitrate. So, no fuel--only oxidizer. Some analysis has suggested that the efficiency of a detonation here would only be .15. Furthermore, probably a low-order detonation (only part of the material detonating). Even actual ANFO typically needs an intermediate booster explosive to achieve a good high-order detonation. For example, instead of just having a detonator on its own a blaster might add a PETN booster charge to make sure the ANFO all detonates. Without that you typically see pellets of undetonated ANFO scattering around the explosion. Hell, even with a booster you still tend to see ANFO pellets lying around the blast area.

All that is to say, this was probably closer to 200 tons of TNT, very roughly speaking.