There is absolutely NOTHING wrong in adopting the core technology of a product which was based on very sound engineering and design e.g. German Dr Felix Wankel designed the rotary engine but it was the Japanese Mazda that perfected it.
Whether it is in the PW F119 or its evolved F135 or the WS-15 variants, there are no doubt that they are all heavily influenced by Russian R79-300 engine and the core technologies are drawn from it.
As for the so-called Technology Transfer from Russia to China, this was probably what happened?
We all know that both AMNTK Soyuz of Sukhoi, they are all Russia SOE and they were all cash-scrapped and needed to survive the crisis. They did not have any new fund for further development, so they were forced to and reluctantly sold off these design to both China and USA in exchange for cash.
So they probably told the Chinese:
"Here are the blueprints and a finish prototype model. Take it and sort it out yourself."
So R79-300 ended up in USA as the evolved F119 and F135 and in China as the evolved WS-15 emei. Certainly NOT a bolt to nut copy as some may like to imagine.
In histories,
NO developed countries have ever really transfer their technologies to other nation.
Yes. The story by the Chief designer sums it all. Yes. Designing and building aero-engine is NO easy task.
For China it has been tedious and heartbreaking learning process. Although China has been involved in constructing aero-engine since the 1950's and it was only quite recent that they achieved breakthrough and success e.g. China is now the second nation in the world apart from USA that has successfully designed and manufactured hollow section crystal blades.
In fact the construction of aero-engine involving many related, strategic and critical technologies apart from good design and a pool of experienced aeronautical engineers, scientists, machinists, technicians, etc. example metallurgy, powder intrusion machine, high capacity forging press, etc. Mind you. No advanced nations will ever sell them to them.
The 10 storey tall 80,000-ton press forge in Deyang, Sichuan. How can any nation hope to be an aerospace nation e.g. Indonesia, etc without the related technology to build one of these very expensive machine?