I don’t know. My guesses are:

1. Ofcom’s system is automated and/or byzantine, so it sends those intimidatory notices regardless of being hopeless or not.
2. Ofcom doesn’t know 4chan won’t pay shit.
3. Ofcom knows 4chan won’t pay shit, but it’s doing this circus to show 4chan is not co-operating with UK law, so there are grounds to demand British ISPs to block it.
4. Some decision maker in Ofcom is worried about losing face if they back off.

If any of those is true, I have no idea.