"...voltage from various accessories (alternator, starter, ignition, etc.) flows through the coolant ..."

Nonsense.

I think what was meant was sneak path through the coolant to ground due to bad main ground, not the entire current from the accessories going through the coolant. I've had a very loose ground strap which caused my instrument cluster trace to burn out while trying to repeatedly start the engine, which just clicked. Same phenomenom.

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I still call BS. A "resistive" ground path would still be better conductor (flow more current) than coolant.

Ground loops are a different "phenomenon".

And, what if my voltmeter had a very high input impedance and I used a zinc probe???