According to an internal document sent to Apple Stores and Authorized Service Providers, “In order to provide the best customer experience, if a customer reports that their iPhone X is having Face ID issues, you may be able to resolve the issue with a rear camera repair.” Now it does seem a bit counterintuitive that the rear-facing camera could be the cause of the problem of Face ID which relies on the front-facing camera.
However it seems that this is because they’re all connected somehow. According to a post on Reddit, user Razerx1, “I was talking to the Genius Bar guy and he said that the cameras are connected. The lenses differ on what they do. The wide angle did the ones that worked and is connected to the front facing camera. The telephoto is connected to the true depth and was not working.”
Apple has yet to officially comment on this problem publicly or launched an official repair program, so it sounds like these are just internal guidelines for their stores should customers come in with a problem with Face ID.