Not rude or abusive per the CoC
I just flagged an answer that literally just said pretty much "The reason for X is because Left is rational and right is irrational", using a lot of randomly picked statements (can't even call them facts, because they are just that guy's personal opinions on which of his preferred ideological positions are rational). The answer had 30 upvotes (thanks HNQ!) because it's popular to bash right wing on SE.
(the answer has other numerous flaws but this violation of CoC is the big problem for me).
As Machavity answered, and from what I've determined after talking about it with other mods, it doesn't rise to the level of 'rude or abusive' as you argue it does. For that reason, I will not delete it as rude or abusive.
Is there a reason this answer is allowed to exist, especially as a "This is what this site is about" banner (aka an answer on HNQ question)?
Yea, that's a different question. By flagging, you have put that question to the mods. I think it's probably better to have a discussion about that in the broader community.
You could say there a number of issues with the answer:
It makes strong claims that aren't backed up by references. For example, that science and academic insight are rejected by a nationalist movement in the US.
It upsets one group (of users).
Now, these two aren't necessarily things we (want) to forbid.
We have talked about references before. Back in 2012 and more recently in 2018, we concluded that references can be great additions, but they're not mandatory.
I think your post here is the start to a bigger discussion. In some cases, particularly when claims are more partisan, we may want to be a bit more strict about requiring sources. But in the end, that's something for the community to decide, and not for me as a mod to tell users after they've answered.
While I don't think we should try to upset users, it may happen sometimes. For example, when we read something we don't agree with, something that's critical of our own beliefs, etc. In those cases, we don't necessarily want to remove the post, because it may have value.
As it currently stands, the main policy for dealing with posts upsetting users is the network-wide code of conduct.
Again, we may want to discuss the issue in more general terms. Maybe the community wants a site-specific policy for this, but that's something we will have to discuss as a community. It's not something for me to make up and enforce on the spot after someone has already written an answer.
Caught in the middle as a mod
As I've hinted at above, I, but I think this applies to my fellow mods as well, are caught in the middle of this dispute. We cannot point to some meta post to say which parts should be changed specifically. We cannot say that the post should be removed outright for some reason either.
I see that you've tried to edit the post yourself while the flag was still open and while this meta post is still being used to achieve some consensus. On the one hand, that might feel right, but on the other hand I hope you see that it can lead to an edit war. We don't want that.
So I'll propose here what I proposed to the other moderators:
we should have a more codified consensus helping (us mods) to pinpoint why we take some action and what can be done to improve a post (in case of removal) to make it fit the rules.
Without that, we as mods end up making judgement calls left right and center, and that doesn't benefit the community. We might be seen to act on double standard, and we will eventually let some users down.
A more codified consensus will not only help us know what moderator action we should take, it also helps users know how their post is going to be treated.