let me start again.

Coolant and pure water do not conduct.

the radiator is not generating anything.
It has become a path to ground because the coolant is conductive, or they tried to scam you and touch the electrode to the radiator itself instead of just the liquid inside.

IF the coolant is still conductive, they did not refil with distilled water as you are supposed to, or did not flush the system as they were supposed to, and you are running around with old coolant.

ANY SET OF DISSIMILAR METALS that are connected with a conductive material, IE actually bolted together, or wires attached, or in a conductive solution will experience some type of galvanic corrosion.
The best way to treat this problem is to isolate the two parts. Since this is not normally possible, the next choice is to put in a sacrificial anode.

Depending on the materials and the amount of conduction, the decay will vary. So some people never see an issue. My bet is those people change out their rad fluid pretty regularly. Or it is so deep into the engine they cannot see it.

Good ground help, but do not prevent. Since it isn't about having a better path to ground, rather the metal's desire to giveup or take an electron from the other metal.

Aluminum and zinc are the two worst in wanting to give up. Fortunatly Zinc and magnesium want to give it up more than aluminum, so you can have a piece of metal you don't care about corrode and die before the radiator or heads you do care about.

It is not that there are 12 volts coming out of the raditor, it is the fact that the radiator is a path to ground when the coolant is conductive.

it is correct to say, that after a while it will be come conductive again. While there are several reasons for this, impurities it picks up is just the simplest way to say it.
Due to circulation and air entering the water, the ph becomes acidic over time, causing more corosion than pure water. It absorbs co2 in the air and becomes carbonic acid. this causes even more "stuff" to enter the water as it circulates.
The coolant is added to prevent this or neutralze the water with its additives ensuring it remains non-conductive for a longer period of time.
But regardless, you will still have to flush the system out periodically to prevent it from being fully conductive.

As asked before, why doesn't he fix the problem instead of the band aid.
Well there may be a few items to make it last longer, but it is not something that can be stopped without the band aid. it is how those items react in chemistry.

Now that is not the same as doing nothing else.
The options were more grounds.
flush with fresh coolant and distilled water
check for electrical leakage
but even with all that, a sacrificial anode at 5 bucks a pop, would be something to add to any system.