Step 1: consider the author and their track record for their past predictions, including their selection of sources (and the track record of said sources)
Step 2: consider the claim itself and whether it jibes with the existing understanding of the topic in question
Except you're throwing the baby out with the bathwater here. Effectively, your entire attitude is that if Minnie Chan, for whatever reason, decides to cite you on a subject, via Step 1, you are now no longer a useful source.
As I've explained before, Minnie Chan has her systemic biases; she can't properly evaluate the accuracy of her Western sources, and she has a relatively weak technical background. But her Chinese sources deserve evaluation; i.e, Minnie Chan's article shouldn't be treated as a gestalt (the article is credible or uncredible) but rather we pick it apart for claims and sources for those claims.
I would say that Minnie Chan's claims that she's gotten Chinese think-tankers to say that the Chinese are targeting 200 J-20s by 2050 deserve evaluation and cross-checking. The claim that the WS-15, from a source that is usually jaundiced against the progress of the PLA, is nearing readiness also deserves evaluation and cross-checking.
From you and latenlazy, you've been negative on the possible progress of the WS-15, when there's actually been an absence of news and insiders have been cautiously optimistic. That's not the same as being able to cross-check Minnie Chan's WS-15 claims as uncredible.
Actually, I would say that for PLA watching, it is standard to value the credibility of online leakers and insiders more than traditional news media.
FYI, I think the issue is that you're trying to take leakers at face value; when I've explained before that leakers face a specific predicament that requires that we subject their claims to the same scrutiny as other sources. As far as distrusting traditional news media goes; I would say that you should distrust everyone to begin with, but give more credence to Chinese news media than Western news media because Chinese news media often falls into your category of insiders, i.e, they have access to inside information. For that matter, Minnie, writing for the SCMP, is somewhere in between, because she's writing for the SCMP, which is traditionally Western-slanted, but Alibaba-owned.
Ultimately, of course, it's a matter of wait-and-see; if we see WS-15 flying during Zhuhai or 1/11, or insiders / official Chinese media exclaiming that the J-20 is running WS-15 (as you and latenlazy have been pains to refute), your hypothesis will have been refuted. If not, this is just another flash in the pan.