True, but it is NOT a key-to-victory (制胜关键) tech. AND engines like WS15 would be impossible (or totally not cost-effective) to copy. Selling engine will give WS15 a much higher production rate. Not that China would need to do this. But it would be nice for the Chinese Aerospace Engine industry for getting larger revenue.
It's kinda like how even-though Boeing and Airbus are in direct competition, each representing a large volume of high-end (high profit) trade for two competing economic entities (EU and US), the US engine industry still strive to provide Airbus with it's best engines. If the US only think about Boeing sales, they could simply cut off (or limit) Engine supply to Airbus and gotten rid of this powerful competitor. But the more engine the US sells, the better their tech gets, and the more money they have to invest in improving their techs.
