Now that you mention it, that's probably why they sent this letter. They know it's pointless but they want a paper trail to show they tried to find other solutions before requesting a block.

It's entirely up to 4chan to voluntarily comply and avoid any further consequences.

yeah but it's 4chan, they won't, the biggest consequence is they get blocked in the UK. And the kind of people that go there will be more than savvy enough tto use a VPN