If it’s not fuel contamination, there aren’t many other explanations for both engines failing.
I agree that an EPA exemption for DEF in airside airport vehicles makes a great deal of sense, at the very least until better safeguards and practices can be developed to avoid cross-contamination. Every dock hand at every marina knows not to put water into a fuel tank or fuel into a water tank, but as long as boats have two holes about the same size, every once in a while some poor schmuck in a hurry is going to put the wrong hose in the wrong hole. And they're usually not doing it in Chicago during a blizzard.
That being said, in this particular incident, my money is on our feathered friends. God willing, the pilots will have a full recovery and can tell the NTSB which one of us was right.
I'm not overly against an exemption however anyone dumb enough to put DEF into a fuel tank headed for an aircraft upload has no business being anywhere near an airport, even as self loading freight. One would figure a job would take up too much of their paste eating time.
Not to excuse anybody who has made the mistake, but it is a lot more of a holes in the swiss cheese lining up than just "putting DEF in a fuel tank." In the circumstances that have occurred, the DEF was accidentally put into the deicing inhibitor tank. This is important for a few reasons:
1) deicing inhibitor such as prist and DEF are both colorless, and odorless so they appear similar
2) the deicing inhibitor tank is usually a similar size and location to the DEF tanks on a lot of smaller fuel trucks
3) Typically both the deicing inhibitor and DEF are in 55 gallon barrels and in spill containment so product labeling isn't easy to see. They are often co-located too so the operators can top off all fluids when topping off the jet fuel in the truck.
4) The DEF being added to the inhibitor tank allows the DEF to be injected into the fuel stream after it has passed through the trucks fuel filters.
5) Both DEF and deice inhibitors are just "dumped" into the tank via hand pump, gas can, etc so there is no female to male connector safeguard or specific nozzles (like a J spout for Jet A for instance)
6) If DEF was added directly into the fuel tank, it would mix with the Jet and and crystalize on the filter blocking fuel flow (same way it causes engine failures) meaning it would cause the truck to fail before it would get pumped into the aircraft.
Obviously now that the threat is known a lot of providers have put in safeguards such as adding witnesses, lock and keys, and segregation of fluids. Jet engines can burn horrifically contaminated fuel and still not fail - DEF is unique that in trace amounts it causes the crystallization and eventual starvation of the engines. We are talking about just a few ounces in thousands of gallons of clean fuel.