That’s not a backdoor in the traditional sense. It is not a function invoked in an undocumented/illicit way, for example by passing a “special value” to an otherwise legitimate program unit which has been illicitly modified to respond to that value.
Might not be a back door in the traditional OS sense like an open port, but a back door for key protection algorithms. That’s because encryption and protection of keys is only as good as the algorithms and anything that weakens the algorithm in any way is a back door.