They don't even specify what sort of shell it is, if it's a regular AP or APCR shell then the worst that would happen is the propellant charge would go off, and to do that you'd require more force than can be applied by dropping them, even if it was HE, HEAP, HESH or some other equivalent ammunition type the chances of the explosive charge detonating just from being dropped is highly unlikely and if the shell is rusted then there's probably gaps for the pressure to leak out of in the event the propellant charge does detonate which, in such a situation, is highly unlikely because the charge is probably damp or otherwise inert
Tank shells are designed to take punishment, they line the walls of the tank for gods sake, they're designed so that they don't detonate unless they're supposed to or a shell hits them directly.
