You see? I, personally, view that as adding to (what Scripture has decreed). But again, don't pay me any mind, it's what God considers adding to, when you consider something other than what He put down in writing. I can't think of any "sources of authority" that would equal His, but then I'm not Catholic (praise God again).
So, Scripture alone matters, and the early Church Fathers and their writings are to be disregarded, as well as reason and the accumulated wisdom and experience of the Church. Yet isn’t it “adding to Scripture” to contrive such a foundational doctrine and claim that it is drawn from Scripture? Indeed, the passage at issue seems better interpreted to mean not to vary Scripture than as a command that Scripture alone may be consulted. Thus sola Scriptura is not really Scriptural but more a doctrinal contrivance that expresses Protestant anticlericalism.