What is a subpoena, if not a signed letter from an agent of congress directing either testimony or production of information? Its possible this letter in itself could be considered a subpoena, since it was delivered in an official capacity. The only formalisms I'm aware of are guidelines and convention, which don't really mean anything anymore, and this letter seems to fit all the definitions I can find. I know it's a dumb question to get hung up on since obviously "talk to a lawyer" is the #1 thing to do here, but still it's an interesting question as to how legally binding an order in a form like this actually is.

What? No it doesn't - the courts can rule on a subpoena once given out, but congress absolutely can issue them itself without certification from the courts. (edit: I'm actually unclear about this requirement, it's quite possible you're correct in that the courts must endorse an issued congressional subpoena) That's a cornerstone of the separation of powers.

Isn't that what a subpoena is? A letter from a duly authorized congressman or comittee directing production of information or testimony?