So… your source to back up your point is an excerpt from a fictional book written by someone who’s expertise is in writing fiction?
Personally, I try not to take the word of someone who is not an expert, or at least versed in that particular area. Just because Pratchett was very a progressive writer doesn’t mean his opinions on gun control should be taken for anything more than his own personal position.
And if we’re just going to cite his fiction as his opinion, we have to assume he was also pro-police violence. I don’t know how much Discworld you’ve red, but even as Vimes progressed as a character and got better in a lot of ways, he always ended up resolving the issue by skirting the actual law and bending the rules to fit his purpose. Often he would espouse how much easier his job and the city would be if he wasn’t restricted by the law. Not everyone else, they still need to follow the law, but Sam Vimes knows better. There were even times when Pratchett would start to push back on that idea like he was going to have Vimes actually understand that police aren’t special and should be as answerable to the law as anyone… then the conflict would always be resolved by Vimes going outside the law and taking it into his own hands. He never learned that lesson. Quite the opposite actually.
So for those unfamiliar, Pratchett was so conservative, he was writing about rogue cops taking the law into their own hands before you kids ever heard those words put together.
Also, only really works if they are “attempting to gain a higher moral authority” (as OP says). As if that’s the only reason people would argue a point. I think it says something about OP that they take that as a given for arguments. I can immediately imagine scenarios that one can argue against a thing that they themselves participate in.
“Hey, smoking is bad, kid. Don’t do it.”
“But you smoke! And I look so cool with a cigarette!”
“Yeah, it’s a habit that’s very difficult to break and it makes your life worse in every way. I know from experience.”
“No you.”
But I agree with your main point,
It’s considered a fallacy exactly for this reason. When you’re debating a thing, you’re way off the map if you think that’s your winning move if you’re arguing in good faith. An argument should be about showing your point is correct, not that you’re better than the other person. But Mr. Wang up there may only view arguments as a competition to be won morally.