I think your dealer service advisor may be misinformed (which is not surprising)..
Link to the trailer hitch part numbers:
https://www.realoem.com/bmw/enUS/sho...diagId=71_1119
Link to a few online vendors where you can get the kit, showing that it is available:
https://www.bmwdirectparts.com/oem-p...10-71606889279
https://www.getbmwparts.com/oem-part...10-71606889279
The cost of the kit seems pretty reasonable. The cost of labor, to include some coding, will probably at least double the cost of the kit.
The question about the X7 coming pre-wired from the factory for the trailer hitch is a valid one since BMW is not always consistent with pre-wiring different options in different models/years. But I would imaging the presence of pre-wiring could be confirmed with a VIN search (either on the net or by the dealer)....