Dictionaries typically only tell you the current common or professional usage, not what someone thousands of years ago meant. And my dictionary shows both meanings without distinguishing who uses which version (as they almost never do).
We are talking about a subtle distinction between what is believed to be real and what is actually real. A person can never know an actual fact because all facts are merely crude approximations, cartoon representations, of the physical reality. Your mind cannot perceive nor conceive of any actual truth. Your brain simply can never be that big, nor does it really need to be. When you look at a tree your mind yields a “good enough” image and you call that “a tree”. But you will never know the reality of that tree, merely a cartoon estimate of it.
Because of that inability to know exact reality, what is said to be real is always merely a cartoon construction, an ontological picture. But when anyone says, “X is real”, they don’t fill in all of the details of exactly how close to exactly real X is. They couldn’t do it even if they tried. To you, X is a tree by what your mind conceives a tree to be. To you it is a “fact”. But that fact is never the total truth.
Thus what you believe to be true, regardless of any verification, is formed merely by the premises your mind used to construct the conclusion of truth. Your mind’s premises cause the conclusion of truth. Your premise of “what is, is what is” causes your fact to be a fact.
But even more, in the one case of that one premise, “what is, is what is”, reality itself, regardless of any belief, really is caused by that one fact and it causes everything to occur in the way that it does. So it isn’t merely a case of some presumed premise in reasoning, but also an undeniable fact of reality.
Because, by the cause of, “what is being what it is”, all things come into the world and behave as they do. That one premise is not merely the first cause of belief, but also the first cause of reality itself. Everything really does happen merely because of “what it is”.
What it is, is what is causing reality to be what it is (both as a subjective reality as well as objective reality), "God creates Himself".