They run way hotter oil temps then a comparable 5.7 does.The majority of piston temp is transfered to the cooling jacket.. The BGE block was developed with the idea it was going to be boosted ,so the squirters were designed in right from the get go,with the permiss of running a seperate stand alone oil cooler,to pull the oil heat generated by squirters down to a managable temperature. The 6.4's don't run a cooler as much as they run an oil heater,that'd be the same as saying the thermal management unit on the side of the 8 speeds is a cooler,when it's an oil heater.Even if you classed it as an oil cooler,it doesn't have enough surface area to cool the oil,especially when you factor in the fact it's passing several gallons of oil through it every minute at highway speeds
One thing is always true, an automotive manufacturer won't put something on a vehicle unless it's needed or the cost can be justified. You won't see a big block engine boosted for a work vehicle so the sprays were put there for thermal management. It's one thing to run hot on a pull down the strip, it's entirely different loaded pulling a grade. Also the article notes the top piston ring is lower on the truck engine, it also utilizes sodium exhaust valves. All points to the expectation of hotter running cylinder temps.
Over cooling the coolant and/or oil can cause issues when the engine is heat soaked under load where parts of the engine will be significantly different then others, this can be bad for cylinders as it can affect the combustion process. It's almost mind boggling how many temperature sensors are placed all over engines when the OEM is testing them.
There have been countless conversations about the oil cooler/heater over on an EcoDiesel forum, if it transfers heat that happens both directions.
With a Hellcat the thermal load can far exceed the cooling system capacity, hence the oil cooler needs to be able to cool just as fast meaning the temperature difference across the cooler has to be higher. The truck oil cooler is more like cruise control, it's cycle is going to be over a long length of time.