I went to an AT&T store that I usually frequent to ask if they would accept my phone, and the person told me that I have to order through AT&T and not Apple to get the credit or I will only get $200 for my X. Told me to cancel my order from Apple and everything to get the $800 credit.
I just called AT&T, and the person said that's not true, of course, and they don't know where that's coming from. The offer says all channels and not only AT&T, of course, and that I meet the requirements for the $800.