I think your therapist made that up. My understanding is that HRT and SRS letters are from one medical professional to another, saying that you have been evaluated against and meet certain criteria, so that the next step can proceed. For what its worth you could have a read of the WPATH Standards of Care, but they don't have to follow that.
To be practical though, there's no point in arguing and delaying your letter. If it were me I'd carefully cover exactly what the therapist said, and write a little bit more than the minimum (I don't like writing). I doubt the endo's going to read it either, especially not if its 100 pages long. The object is to keep the therapist happy, not follow some standard medical rule.
I'm assuming you're not living somewhere this actually is policy. I don't think it is where I am.