This was a wild guess on my part. What do you think of his other assumption, of oxygen defining acid?
Your answer will be available when you show us the oxygen in Hydrochloric acid....
Note that the very name, "sharp-maker", reflects this observation, and it certainly corresponds to a variety of acids. He had the advantage of the atomic hypothesis, so his systematic method carried him very far, as we appreciate.
Of course, he overgeneralized the significance of oxygen, and his conception foundered on "muriatic acid" or HCl. He assumed that Cl must be an oxide, XOn, and it's impressive enough that he observed of the presumed "acidifiable base", X, and Oxygen, that "... no method has hitherto been devised for separating them".
Again, the point is that he was not doing what he thought he was doing in his method of interpretation, i.e. reasoning deductively from experiment. He was conceptualizing and testing his concepts against experiment, and in terms of any governing "precise rules", this conceptualizing step is the wild card, and what gives life and power to the whole process.