I, without proof, think that ofcom will file these letters as "second best outcome" (from their perspective).

Of course it would have been "best" (again for ofcom) if 4chan had just complied but I don't think they ever expected that.

With these meaningless letters (for the reasons you state) the agency can later go to law makers and ask for more domestic enforcement power because clearly "it's not working".

If by chance one of the 4chan people stumbles about that personally they'd consider that as a bonus.