The actual authentication & crypto is happening inside the main SOC. The Face ID sensor is (or should be) just a dumb camera - there's no reason for it to be part of the chain of trust.
IMO the security of the system is that the 3D face data is reasonably secret (you probably can't reconstruct it from a single picture, you'd need pictures from all angles) and the SOC only allows a few attempts before falling back to passcode.
If you have the face data, you don't need to tamper with the hardware, and if you don't have that data then tampering with the sensor won't get you anywhere.