We are a long way from knowing if structural failure is involved here, but smooth air doesn't rule anything out. A structure on the brink of failure is just that - a structure on the brink of failure. An aircraft in flight is sustaining loads greater than "static" when it's sitting on the ground. If the structure has been compromised, it will eventually fail under "normal" flight loads. The only thing turbulence will do is speed up the process.
Things will fall off just sitting on the ground due to corrosion and other factors over time. However an in-service aircraft of this age will have no doubt been subject to regular inspections and NDT of the critical parts of the airframe for cracking, corrosion and anything else that will lead to break up. Previous accidents involving the type would have generated ADs for the parts most susceptible to these failures, so a failure in straight and level flight at cruise speed even in moderate turbulence is very unlikely if the aircraft was properly maintained. And once again, the ADSB data points to a gradual slow down to close to stall speed, under control, in straight and level flight, very unusual if it was a wing falling off. Excursion from cruise flight only seems to occur once minimum speed is reached and the aircraft then departs into a left spiral. This is shown in a trend in the data, not single points, so it's doubtful the ADSB plots are all in error, as all the plots prior seem relatively accurate and the departure coincides with the loss of contact with the aircraft.
I also don't really believe an experienced pilot would not note an engine failure and let it get to stall, in a training exercise even less likely. I assume multiple bells and whistles would be sounding at various times, being a turbine powerplant. There was also at least one other aviation professional on board in the case of the engineer, who would have had some clue of what was going on had an engine stopped.