On the other hand, there are many reports that Leonardo himself lamented not finishing things, and wrote “Tell me if anything was ever done” in his notebooks.

On the other hand, there are many reports that Leonardo himself lamented not finishing things, and wrote “Tell me if anything was ever done” in his notebooks. (I don't know the full context of this—searching online suggests the original was "Dimmi se mai fu fatta alcuna cosa"—but as you yourself say, Leonardo was prone to perfectionism and starting far more than he could finish and had a reputation for disappointing patrons, which seems to fit well with the author's main point, even if the example of the horse doesn't.)