Hi,
electrolysis really is an electrical reaction between two dissimilar metals accelerated by and electrolytic solution such as water, or any liquid really, some more a reactant than others, varying with salinity or acidity as well.
Alloy heads have been on cast iron blocks for yonks and did/ do still utilise copper heater cores. (price of copper slowing that up tho)
Some engines such as Datsun 200B (L20 I think) and Mitsubishi sigma (circa 4g54 and etc) are very renowned for their head munching ability. This can be corrosion and electrolysis with hard water. Other vehicles you may have owned in the past may have chewed away thermostat housing or the heater outlets on the water pump.
There are several ways to minimize electrolysis.
The first used and then traditional method was to supply a sacrificial anode, which is a earth point for the reaction. Thermostat housings are (were) pretty quick and easy to replace so it was often made out of an appropriate (shi*ty) material and died for the cause. A cheap sacrificial anode to prevent a alum. fishing (tinny) boat is to hang an old carby off the boat.
The next more modern method is good coolant. Coolant is more of an art nowadays, with ethylene and propylene (and prob many others) coolants are more made to suit an application now, much like engine oils. From watching them evolve over the years you can see a good coolants results when you dismantle an engine. Water galleys are clean because they have had a nice coating on them.
Dare I compare it to spray on non stick for fry pans? 
Water filters on engines heavy diesel engines are often impregnated with 'units' of DCA. These engines might start with a DCA level of, lets say for eg, 12 units. So, we get our theoretical engine in a theoretical tractor and set it to work, after about 500 engine hours the heat and movements in the engine reacting on the coolant has depleted the DCA level to 8 units, you can easily measure this with a refractometer or use a paper dipstick much like a urine drug test strip. Screw on a new filter of 4 units (they come in different numbers to suit) and all's back to spec. You can pour a 500 ml bottle into the coolant system to replenish it also.
Without DCA coolant protection the liners in an engine may end up with pin holes in the side and water leaks into the piston swept area. John Deere 40 series tractor used to do it so close to 10,000 engine hours you could set your watch by it.
The pin holes are not caused by corrosion or electrolysis however, they are caused by cavitation, which sadly is a whole other encyclopedia, but to cut a long story short(ish) the up and down reciprocating thrust naturally produces some sideways thrust on the cylinder walls, they shake like sh*t and get vacuum pockets beside them and the water as they seperate momentarily. As the vacuum implodes it focuses to a final point of impact that "chips" a little bit of metal out of the side of the cylinder bore on the water jacket side. These eventually eat thru and a re-sleeving of some sort is required.
Coolants ain't coolants, they are pretty vital and should be maintained depending on your needs really, if you had a pool I'm sure you'd Ph it or whatever pool people do.
I also agree that old can also be corroded, I know my old man and his mates never had the cooling system preservation options we have now. Another validation for the comment you heard is probably in the fact that earlier cooling systems often used pressure caps around 7 - 11 PSI which eventually standardised on 13 PSI for a while. Older Smiths systems are brilliant heaters as used in many of the Rootes Group. BMC etc cars. Good olden day stuff, where an overkill was the way to go and quality was affordable. Probably a good choice to source a semi-freestanding unit for those that take the donor car route like me.
I got some unfitted funny asian produced heater unit with my roller, reminds me of some Nissan Cedric item for some reason. Gonna be a ***** to mount, dunno why we bother
, it's Queensland, who needs a heater
Sorry people, I'll try not to do it again.
Note to self : short mate, SHORT FFS,