It says "we are hereby transmitting a subpoena". When he says "this subpoena" in the next paragraph, he's referring to the subpoena that is being transmitted, not the actual document you're reading. The same language "this subpoena" is used in the letter to Epser, which is clearly not a subpoena since it concludes with "the enclosed subpoena".
It's totally standard to include a letter explaining what the subpoena is all about since the actual subpoena doesn't have much detail.
I've been subpoenaed.
Oh good lord. So are you telling me that his reams and reams of posting about this thing can be summed up by as him deflecting to the cover letter as opposed to the other piece of paper which is the enclosed subpoena? He could have simply said that and ended the entire topic? I do note that the other letter also had enclosures. So much wasted time.