The way I figure, if a sophisticated attack is so sophisticated that it can only be state-sponsored, then it is sophisticated enough to appear to be sponsored by another country.
Your problem here is ability and motive. What other countries are capable of executing this attack and would want to steal this information? As far as I know Comac is the only company that is attempting to build planes that rival Boeing and Airbus for commercial flight so you have your motive. The Chinese have a history of corporate espionage (lookup Huawei and Nortel) and given that Comac is a state owned company yo…
Boeing also competes with Airbus, as in actively competes in real dollar terms compared to Comac which is more or less aspirational. Given their current situation they have as much incentive to hack Airbus, plus they've been accused recently:
Airbus to sue NSA, German spies accused of swiping tech secrets
>European aerospace giant Airbus is promising legal action over claims its top blueprints were stolen by German spies and given to America's intelligence agencies.
https://www.theregister.co.uk/2015/04/30/airbus_us_german_in...
My view is that both Boeing and Comac is actively stealing from airbus because why wouldn't they?