It's interesting that Delta's testimony didn't contradict Boeing on the most important allegation: price. Delta's assertion that Boeing didn't offer an aircraft against the CS100 is a red herring.
Boeing's proposed scope in the anti-dumping complaint is the class of aircraft with "standard 100- to 150-seat two-class seating capacity and a minimum 2,900 nautical mile range". This effectively lumps the CS100 and CS300 together in the "domestic like product" analysis. The CS300 competes with the 737-700 and 737 MAX 7.
In the original complaint, Boeing went to great lengths to argue that the "100- to 150-seat" category is the appropriate grouping, citing prior findings and decisions from the European Commission, the Government of Canada, and even Bombardier's own marketing materials.
The bottom line is that Boeing anticipated this line of defense and Boeing's attorneys pre-emptively crafted the complaint to tie the CS100 and CS300 together.