I disagree with this.
Parsimony selects the tree which represents the set of least falsified grouping hypotheses.
An ad hoc hypothesis is one which addresses a particular case and seeks to explain why it is incompatable with a more general hypothesis. The general hypothesis is homology. Realists think there is a truth which we are trying to describe. It is not necessary to assert that the best hypothesis is actually converging on the truth, although we obviously try to develop our science in such a way such that it does. Even when the one tree has a low probability of being the truth.