From such a perspective, each transformation is a putative homology, a hypothesized evolutionary event. As such, all have equal evidentiary weight. I find Felsenstein's point to be a rhetorical gambit to shore up support for a probabilistic approach to phylogeny reconstruction. In fact, a parsimony-based approach makes no such implicit statements, for it does not operate in a probabilistic framework. And he certainly knows that.
Probabilism seems to run into at least two fundamental limitations.
The need, in phylogeny reconstruction, is to pass some sort of judgement on unique, singular events, and these either occurred or did not. Their calculated probability in the context of presumed knowledge of general processes is really irrelevant to the question of whether one particular instance occurred or did not.