*I know lawyers aren't free, but he'd be paying a lawyer regardless of the letter's style.
I speculate (at the risk of demonstrating my ignorance) that this letter, at 1 page of background and ~4 pages of analysis, may not actually have taken all that long to prepare, especially for someone with intimate knowledge of DMCA and libel/defamation case-law.
Of course, it's a lot more complicated if he has arranged representation rather than just letter-writing, and the contact telephone at the end perhaps suggests he has, so it's quite possible I'm wrong.
I do take your point that it is good publicity and on a related note, it is an interesting tactic to publicly disclose the legal response. It obviously will feed into the public support that the Oatmeal is receiving.
I'm not taking anything away from the letter itself - it was a good read - it's just that I don't think FJ's letter warranted or indeed deserved such an in-depth response.