I'd buy that if this was a WTO ruling but these are tarrifs adjudicated by Americans, supporting an American company.
The US similarly imposed tarrifs on softwood lumber not that long ago, and they were determined to be illegal once it hit the WTO. There's no reason to believe that won't happen again.
* Boeing is guilty of the same, selling hundreds of both the 737 and 787 far below production costs to be able to compete with Airbus' 320 and 350.
* Slapping such a massive tarif onto the CSeries was a political move aimed purely at squashing a new competing product that barely overlaps in cost/capabilities with a 40 year old, obsolete Boeing product.