I hope I don’t sound rude but it really sounds like you only consider WaPo trustworthy when it’s convenient for you.

That's not quite what they're saying. They're saying WaPo is also subject to censorship and coercion, so their word holds more weight when it's a topic where they might be penalised for publishing. If you don't think there are any Chinese sources that can publish things critical of China, then you can still follow carl_marks_1312's methodology in part by finding articles from sources with a free press but geopolitically aligned with China.

Sorry, I don’t understand how that makes this any less trustworthy?

To us, Adrian Zenz automatically means you can dismiss the evidence. The person you're talking to went in with that assumption, and then was lambasting you for not noticing such an obvious and glaring problem with the article. So that's where the disconnect comes from. Of course without that assumption the comment doesn't make sense.

Zenz is a garbage person, but more importantly he's not reliable. He's verifiably been caught lying several times. The tweet you commented on is out of context. I don't know what the context is, it probably doesn't change what's being said, but without reading the context I can't know if it's a justifiable thing to say. Perhaps it was. Perhaps he was explaining Nazi mentality without trying to justify it. It doesn't matter. Zenz is a bad source because he's a liar primarily. He uses bad science and statistics, he makes wild inferences, he pretends not to notice mistakes that he must've noticed, etc. He only ever cites circular sources. That is to say media reports of his own publishings.