The Apple iPhone 15 models all come with a USB-C to USB-C charging cable. Is it possible that one can come, directly from Apple, with the older USB-C to Lightning cable ... sure, but it should be a rarity.
It also could be possible that this phone is new, but repackaged by AT&T.
Apple uses different designations for an iPhone's model number for refurbished phones. iPhones that have been refurbished by Apple or a cellular carrier, would have a model number that starts with a "F."
You also have the option to enter the phone's serial number at this Apple site to find out.
Ref: How-To Tell If Your iPhone is New or Refurbished - AppleToolBox