I mean yes, the firmware guards the encryption keys, if the firmware is corrupt then access to the key is corrupt.
If the key could be recovered with a corrupt firmware, then the SEP would be open to an attack to extract the keys by forcing firmware corruption and then using that as a path to compromising the device.
you back up the data - the security model for an HSM (e.g. the T2) is that secrets cannot be extracted. Once the key can be extracted it then that security model is broken.