4chan is a US company that has been delivering services to people in the UK and getting revenue from that traffic. UK law says that if you deliver service in the UK and meet certain criteria then you need to verify the users' age. They didn't comply, and the UK said "If you continue to deliver services into the UK that don't comply with the law then we will sanction you with both fines and blocking your traffic." They said "Screw you" to the regulator. Now when Ofcom asks UK ISPs to block their traffic, Ofcom is an "industry-funded global censorship bureau" and "Ofcom's ambitions are to regulate Internet communications for the entire world, regardless of where these websites are based or whether they have any connection to the UK,"
They're having a laugh.
If they deliver service in the UK then the service delivered into UK needs to comply with UK law. It's pretty simple. It's not like there's no internet geolocation technology. Most of the other sites that are covered by this law have worked out how to deliver UK-specific age verification. It's not that hard.
Personally I think that the UK's new law is ridiculously misguided and broken, not to mention unlikely to achieve what it sets out to do, but telling a business that their service in the UK needs to be compliant with current UK law is not an attempt "to regulate Internet communications for the entire world". They are welcome to join all the other people who are lobbying for the law be repealed. In the meantime they need to comply or cease local service.