Yeah you are right. The higher the flash point the more the oil will conduct heat away from the part being quenched. Higher flash point = faster cooling.
When quenching in water a steam jacket will develop around the part. Preventing the part from cooling rapidly.A little swishing is involved to break up the steam jacket.

Commercial heat treaters that do batch parts will use a pump to keep the fluid flowing.
Fast cooling would be important to get the color just right. That and practice.

Very interesting stuff though. The other fact I pulled from a Machinery manual on Carbonia oil is the ratio of carbonia oil to charred bone was 8 quarts bone to 1 pint of carbonia oil, well mixed while warm.