The engines require the merging of many technologies. This was one of the paper’s points and none of that stuff can easily be copied. It is also worth noting that Russian military jet engines, although superior to Chinese engines, still lag behind US designs and Russia has been in the game for a long time.
One difficult technology that the paper didn’t mention was the Nazi V-2 program. Even with designs and the Germans that were part of the program to assist, both the Russians and the Americans required significant time to duplicate the Nazi effort, even with massive resources and not having to deal with two huge armies bearing down on them.
Look at the PLAF J-20 as a perfect example of John’s point. They’re flying with the WS-10 Engine with protruding exhausts and excessive thermal signature. They haven’t managed to get the WS-15 airworthy and beyond testing.