I get the view that when life gives you lemons you make lemonade, but come on...
Demonstrating capability is much more important than just eye candy. I read your exchange with Justin Bronk on Twitter (I particularly enjoyed how you backed him down on the LO/VLO distinction) and while I agree with everything you said, his ridiculous view has a leg to stand on in part because of the J-20's anemic public displays. If casual observers only ever see the J-20 do turns that make a crop duster look agile, it's not unreasonable for them to conclude that that's because it can't do better. Not everyone is going to read Song Wencong's paper and reason about the inherent agility of the canard+LERX planform.
There were even rumours that amateur video of a J-20 doing a cobra was scrubbed from the Chinese internet; that's just ridiculous.
I'll admit that a large bulk of my motivation is that I want to see bastards like Bronk repeat "the J-20 is an all aspect VLO air superiority fighter" like a catechism, but there are perfectly rational reasons why excessive secrecy is counterproductive, even dangerous. If you obsessively guard every little detail and give no glimpse whatsoever, it's rational for people to conclude that you're so obsessively secretive because there's nothing there. You don't show anything because there's nothing to show. You have no confidence in your capabilities. That can lead to a dangerous underestimation of your capabilities and your enemy might calculate that you're a pushover.
That doesn't mean you reveal every secret, but there's a balance between secrecy and signalling capability and I don't think China is getting the balance right. In fact, I worry that it's getting it dangerously wrong.