Don't know what you're saying. The "letter" he claims is not a subpoena says it's a subpoena and is authorized by the committee via all those sorts of signatures.
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.