The default resolution for the DTM is indeed 5xGSD, whereas the resolution of the DSM is 1xGSD. During algorithm development, we empirically found that the results for the DTM are better at this (lower) resolution. Also, in general, the DTM does not need to be of the same resolution as the DSM to describe elevation changes in the terrain. One drawback of a higher resolution is that a denser DTM will be noisier. Thus, it is a pre-established feature that cannot be changed, the value needs to be equal or more than 5 GSD.