So say I go out rather than dealing with ATT support and buy a 640 to have as a backup and its super cheap and give them that IMEI to program sim and then throw that sim in my xl it will work?
That is correct. As long as AT&T has a branded AT&T LTE IMEI listed on your account, LTE will work on the XL. Do not tell then anything about the XL. They don't need to know you have this phone (or any unlocked device). Telling them you have the XL will only create confusion and break the data provisioning once they enter the XL IMEI (because it will come up as a generic unlocked 4G phone). If you give them one of the two XL's IMEI numbers, your phone will not get LTE, it will be stuck on 4G.
If you don't want to buy a phone you may not use, just to to WalMart and find an AT&T 640 package. Look at the bottom of the package and take a picture of the IMEI. Call AT&T and give then that IMEI. It doesn't affect anything (i.e. it won't "tie" that device to your account, it only enables LTE provisioning).
This needs to be clarified on the front page. It's a really simple solution that's being muddied with confusion. I don't know why AT&T's system is so particular about this kind of thing, but it just is.