[Off-topic]
Personally I think that your "I don't understand" sounds disingenuous, since the rest of your comment shows that you understood the comment that you were replying to. Beyond that, I don't think that you're being impolite, a jerk, hostile, or touching a "forbidden" subject.
Given the four downvotes, other people may or may not found something else that they don't like in your comment, I can't speak for them.
‘I disagree’ is too simplified definition and does not represent my view.
"I partially agree", "I partially disagree", "I think that it's complicated", there are multiple ways to convey this. Or simply going straight to the parts that you disagree with, without the "I don't quite understand".
I know that I'm being obnoxious with this, and I apologise for that. It's just that people who use[d] Reddit - including me - often bring its obnoxious culture into Lemmy, often not noticing it. One of those is to disguise disagreements as lack of understanding. I can go deeper on that if you want.
[On-topic] The whole "punch up, never down" thing is about acknowledging that sometimes you need to oppose people. And it's morally better to oppose the ones "up" than the ones "down". That's it - in some situations it will break, but:
- since you're expected to behave nicely by default, it doesn't justify underprivileged people acting poorly towards everyone else
- it still gives room for people in power to criticise others, specially in defence of people with less privileges (note that "fringe extremists" often target vulnerable groups and individuals)