Hi! Xfinity Mobile user here. I pre-ordered the iPhone 13 Pro without having to select a carrier. There was an option for no carrier at that time. I do not buy my devices through a carrier.
When you buy an iPhone through Apple for Verizon or T-Mobile, you get an unlocked device. AT&T devices are carrier locked. This is stated on the information button under the three carriers during the pre-pre-order setup.
The trouble with this is that it wants to verify an account with one of them. You can't just select one and keep going. I hope they add an option for no carrier.
Update: there is an option for no carrier if you purchase instead of finance.