Thus Goff understandably suggests it is a precondition. As a set of therapy conditions, it seems to me that Rogers is asserting that all must be present to some extent, thus they are all conditions for therapy to happen.
None are preconditions for the others, examined one at a time.
Incongruence of the client. If it exists without the 6th condition, then it is purely subjective to the therapist and unsupported by the client. It is still subjective to the therapist. It may be correct, although unsupported by evidence from the client. It may also not exist at all, although any or all of the other conditions are present.