Look at the radiator mount design. Brass or aluminum radiators are mounted in rubber isolators or other non-conductive manner, so they are electrically isolated from the body and engine and the rubber in/out hoses are also insulators.

Mounting a modern aluminim matrix/plastic side tank radiator by the side tanks also electrically isolates the radiator from the rest of the car since plastic is an insulator.

Electrical isolation mounting prevents radiators from combining with engine castings - either iron or non-ferrous materials - from developing and anode-cathode relationship in the presence of an electrolyte - the coolant.

As stated, the claim that a battery tender could hole a radiator due to electrolytic corrosion is pure, unaldultered BS!

Duke