Exactly...
If we think about an engine and where items are under pressure, oil will almost never get into coolant. Oil is under pressure from the pump, through filter, bearings, lifters and pushed up into pushrods. After that.. it's drips back to the pan, and has zero pressure. Oil does not enter the heads under pressure.
Coolant meanwhile flows through the heads, block, headgasket area, LIM, Alt bracket, timing cover, WP, hoses, heater core and radiator under 16lbs of pressure constantly.The chances of oil having more pressure in an area where it could potentially meet coolant ... doesn't exist.