Blows my mind someone actually unironically makes this argument.
4chan is showing ads on their site, but if your idea had any grounds, the issue would be with the ad network, not 4chan.
While that'd be a pretty bad legal precedent too, it'd at least be coherent.
More realistically, 4chan will likely be banned by UK ISPs after a court ruling.
The previous mail was likely just to move the process forward to show they have no interest in following the UK law.