I’ve read on Weibo that the unit in question operates the J-16, not the J-20, but even then it isn’t like the J-20 is invincible so there is a chance of it getting locked on and vice versa. The document in question is actually a commendation on the J-16 operators so there are some interesting tidbits that the guy sharing it on Twitter, a PLAAF hater, has omitted. Supposedly there is an excerpt on J-16 identifying and locking F-35 using passive (IR sensors) that “mysteriously” got redacted by the poster.
The reason that he blotted out the name of the fighter is because he got butthurt after someone on Weibo fooled him into believing that x-ray scans of a toy J-20 swallowed by a toddler where EOTS footage of an F-35 acquiring lock on the J-20. He hoped to fool people into thinking that J-20 got locked. But even if it does, I don’t see what the big deal is. Getting locked in peacetime is a fairly common occurrence.