Actually, while HN is certainly strongly pro-capitalism, I regularly post lengthy comments here about socialism and have gotten plenty of upvotes from comments that have quoted things likethe Communist Manifesto to make my argument.

It's more likely to have been flagged because there's always some that flag political discussions of any kind not directly related to tech, and even more that flag political discussions that they expect will get heated because they don't think those kind of discussions belong here.

I'm strongly left wing and have not noticed any particularly strong bias either way in terms of what type of political articles get flagged. Specific subjects are more likely to be a trigger.