The case is excluded because the polynomial cannot be minimum phase in that case, because then it would have a zero at unless all its coefficients were zero.
Note that every stable all-pole filter is minimum phase, because stability implies that is minimum phase, and there are ``no zeros'' (all are at ). Thus, minimum phase is the only phase available to a stable all-pole filter.
The contribution of minimum-phase zeros to the complex cepstrum was described in §8.8.
Maximum Phase Filters
Definition of Minimum Phase Filters