The force generated by a solenoid can be very powerful. I presume that the forces are multiplied by the leverage built into the derailleur so we are likely dealing with a very powerful little unit -- certainly one I would keep my fingers away from.

A standard derailleur only has the force of the return spring which is comparatively weak. The fact the someone can move the derailleur in and out by hand shows just how weak the spring is. It is fine in normal circumstances, but when the chain is under load, the spring isn't strong enough to exert the sideways force and the derailleur just sits on the chain.