U.k(BAE Systems) has experience in building stealth/with generation fighter way back. You haven't heard of BAE systems replica which a prototype model was built and rigourously tested way back in 1994 to the early 90s and was intended to replace our Tornados fighters. Unfortunately, due to financial constraints we didn't push forward to full mass production.I believe Turkey can make use of our experience and full designs to speed up it's TF-X program. The model(S) of replica we built are still highly guarded here in Britain by BAE systems in Lancashire though and we are using it as a base(Taranis was inspired by it) for our new long range stealth bomber which is being developed/built with France .
https://theaviationist.com/2014/02/25/mysterious-bae-replica/
BAE Tatanis is based on some lessons learned from BAE Replica.
Plus New long range stealth bomber we are building France
Plus dont forget BAE is the only their 1 company building critical systems of F-35 ALONG with Lockheed Martin. BAE has alot of experience in building fifth generation fighters. Many other british defence companies are also involved in building F35 aircrafts key components apart from BAE Systems who builds the lions share, there is also Martin-Baker, SELEX, Cobham, Ultra Electronics, UTC Actuation Systems and Rolls-Royce just a few british based companies who contribute significantly to his fighter as well.
Think the post some Turkish members were trying to make is that countries like U.S, U.K, Russia and even France have a longer history and experience in aviation/aerospace than China who is fairly new but is advancing/catching up very fast indeed. .