I feel too many people conflate /pol/ with the whole website.
I believe that's fair. Sure, it's "a different board" but it's just another URL on the same domain and same administrator, just different janitors. So it is really the part of the whole website. I know that 99% of people on 4chan disagree with me because they do not wish to be associated with /pol/ /b/ /gif/ but if they wanted to disassociate themselves with those boards then they should be on an entirely different domain without 4chan in name. polchan perhaps.
Do people also treat Reddit the same way?
They do not. Reddit is a big corporate social media site and largely gets a pass in online discourse despite the horrible communities that do and have existed there.
I don't know. I've never created an account there. In it's early days it just seemed like they were trying to make a platform that could be monetized some day so I never bothered. I assumed incorrectly that it would just fade away.
If that is the case that might explain why so many on 4chan feel that different URL's are different sites. Most of the current members seemed to have shown up from Reddit. Most of the original members grew up and left, myself excluded. I still visit from time to time but don't stick around long as most threads and posters are obviously just 4chan-GPT and people being tricked into replying to it.
There are certainly overlapping circles between Reddit, 4chan and HN. 4chan people talk about and make fun of members of this site all the time. They also make fun of Reddit but don't seem to call out specific people on it.