The real favour is not taking you to court for attempting to break into critical national infrastructure. The shirt is just a fun gimmick. Beats only sending a letter, in my opinion.

Also, it's unlikely that they didn't just ask for the author's size and address. The standard disclosure process involves a temporary non-disclosure and negotiating if/when publication of the issue will take place.

The author was pleased with the response, and the process has improved since this was published.