Boeing's complaint is actually factually correct and pretty much rock solid. BBD is indeed being subsidised via tax breaks and sweetheart loans from its various interested governments, and it did indeed sell the airframes to Delta at well below cost. The trouble is that Boeing does exactly the same thing all the time.
These kind of tactics are, regrettably, par for the course in competition between Boeing and Airbus. I think the main reason everyone is pissed off at Boeing is not only that the C300 is inarguably a profoundly superior product to Boeing's ageing cash cow, but they're employing their most ruthless "nuclear" legal tactics against a company so much smaller than them, in a friendly, neighbouring country, which is a customer to boot! At the very least, it's not very gentlemanly. It's pretty rich to shriek about "predatory behaviour" from a company a hundredth their size.
For anyone interested, the public version of the complaint is available here: https://leehamnews-5389.kxcdn.com/wp-content/uploads/2017/04...