> They are stating that companies operating in the UK

They are not operating in the UK. ISPs in the UK have chosen to make content from the USA available in the uk (or more accurately, do nothing to prevent it being available)

but ISPs do not have the power to decide if something is infringing a rule or not.

Thus Ofcom goes through the motions of telling 4Chan "hey I think you're not compliant" and if 4Chan says "lol, I'm not serving UK people" _then_ the UK authority will tell the ISPs to block it (and the will be on the hook if they don't).