I have never understood this concept of "disproportionate force", not during a terrorist-hostage situation. While the Russians used force ineffectively in the end, the problem was not that the amount of force was disproportionate, the problem was that the force failed.
It seems the phrase “disproportionate force” was from the BBC. That makes it sound like someone complaining about police being a bit rough. The verdict itself uses different language - “Though the decision to resort to the use of lethal force had been justified in the circumstances, such a massive use of explosive and indiscriminate weapons could not be regarded as absolutely necessary” and “powerful weapons such as tank cannon, grenade launchers and flame-throwers” had been used to free the school, contributing to the high number of casualties.