I agree with you. In fact, I agree with you so much, that if you look closely you will see I actually said the same thing as you!
(Although, FWIW, if you specify both MOBI and AZW3 as valid E-Ink Kindle formats, and send an EPUB via send-to-email first, then it will send the existing MOBI rather than re-converting to AZW3. Actually, given that I don't really use the PDOC service since I prefer sideloading, it doesn't really matter... to me.)