Cepstrum calculus disambiguation
A common technique for computing the inverse fft is to invert the imaginary part of the input array, perform a forward fft and then invert the imaginary part of the output array. In the case of the Cepstrum, the input to the ifft is real-valued (due to the absolute value function) and also symmetric. In this case there is no imaginary part on the input to invert, and the symmetry implies that the ifft will produce no imaginary components on the output that would need to be inverted.  Hence in this case the fft and the ifft are fully equivalent.