That's like saying that my car has a refrigerator because I can buy one separately and put it on top. There's a substantial difference between "having it" and "being able to have it." The former looks at the device itself while the latter can include add-ons. If I buy the AT&T 1520, I cannot choose the case of my choice because I'll have already have to attach a QI shell to the phone, something that buyers in other countries won't have to do.
For security reasons, WP treats SD cards very differently than internal memory. They are not equivalent at all. Also, internal memory has higher performance characteristics than SD cards.
AT&T hobbled a flagship Windows Phone. Whether this was done to preserve an Android or iOS advantage, to increase profit margins, or because someone goofed in picking features, the end result is the same: the AT&T version of the Lumia 1520 is not as good as anyone else's and is a step backward from the 920 in two features important to many users.
I don't know that I want a 6" phone, but I know that I don't want that one -- even if AT&T offers to give it to me. If I really wanted it, I'd be looking at unlocked ones or versions from other carriers. I'd give up LTE before QI or 32GB.