Who cares who knows more?
All I know is Russian is developing Checkmates to kill J-20s, so exposing even the slightest secret to a Non-Ally nation is risky with little to zero benefit. It's a childish way is gaining friends. Do you see US courting India by sending F-22s to QUAD exercises? Because US isn't as dumb and careless with self-defeating strategies.
That's not the way fighter development works.... it is rare to develop platform for platform counters to an aircraft, especially in this day and age. Any useful information that can be gleaned about how a modern fighter operates, would be the signals and RF information and electronic order of battle data that would be applicable for EW, networking and sensing, and that is not exactly platform unique.
Now that we have out of the way -- there are many ways for nations -- even close treaty alliance nations -- to preserve those secrets from being exposed to each other in military exercises.
The Sukhoi LTS is an interesting project, but you are overreaching when you're saying it is being developed to kill J-20s. It just doesn't make sense.
Furthermore, I would argue that as far as trust goes, Russia and China are both fairly close. Certainly close enough to allow their top end fighters to be involved in the same exercise to some extent.
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I'm going to give you some advice -- the line of argument that you are making is incorrect and it's not how it works.
Better for you to walk away now than for people to crowd over you to explain it multiple repeated times.
If your argument is about the nature of the Russian and Chinese partnership in general, then that is an entirely different topic, because this entire discussion underlying the involvement of J-20s in this exercise over the last few pages was made with the consensus that the relationship between China and Russia was one of a significant and growing strategic partnership.