While I have no specific knowledge on even jet engines, from general engineering principles, who says you cannot have 2 groups of people, one working on your own design, the other working on reverse-engineering / replicating others' works? For critical projects like this, it's not hard to imagine a State Actor throwing at least this much resources (including human resources of course) at this. One benefit I can think of is that reverse engineering can shine light on some tough problems during the "forward engineering", like "can this be done at all", and "what is the major concern / bottleneck when designing a particular component". Even if you don't figure out the whole thing, the process can still be useful, especially when you don't already have all the know-how.