Not quite. There are well-established legal mechanisms for Ofcom (or anyone) to try to engage legally with companies domiciled in the US and with no locus in the UK. Rather than using these mechanisms, they have tried to short-circuit the process by sending emails that have no legal force.
Hmmh. If some powerful law enforcement agency was coming after me to stop my website, I sure would hope they would first send me an email asking me to stop.
The US-UK Mutual Legal Assistance treaty imposes obligations on Ofcom which they have not met, 4chan claims:
“None of these actions constitutes valid service under the US-UK Mutual Legal Assistance Treaty, United States law or any other proper international legal process.”
MLAT applies only to a narrow set of legal procedures, essentially around criminal activity. I’m a lawyer but this is very specialist stuff. I’m not expert enough to opine on whether MLAT applies here but - simply judging by the quality of their respective legal work on display - I’m minded to believe that Ofcom knows what they are doing. OTOH 4chan’s rhetoric reeks of FUD.