Marchionne stated the JL is locked in and finalized many months ago (well before these mules were spotted). No reason to be testing diesel otherwise. It's not like these decisions are made and changed on a dime. Takes years of development and testing. Seeing a JL mule testing a diesel engine this late in the game is as good as done and their own internal docs have the diesel slated for 2019. Like I said it's fine to be skeptical but you are betting against a lot of solid evidence at this point.