Note that while the US does not recognize a US based server as being under foreign jurisdiction just due to being accessible in that country, there are also some subtle rules that can cause it to be considered to be doing business there by the US legal system, even if they don't have a physical presence there and are not directly selling product there. One of those is targeted advertising to people in that foreign country.
I don't know whether selling ads aimed at people in a country qualifies. It may not, or that bit might not have been hammered out by courts yet, but if I were 4chan, I'd be really careful on that, as they're explicitly mentioning that they have a British userbase on that ad sales page:
Location: United States (47%), United Kingdom (7%), Canada (6%), Australia (4%), Germany (4%)

If the US legal system recognizes that a company in the US is doing business in the UK, then the US legal system will view the UK legal system as having jurisdiction and enforce rulings against them from the UK's legal system.
4chan's argument here is going to be that they don't meet that bar. I expect that 4chan is most-likely going to be able to successfully make that argument, but the "doing business" bit does matter.