And I agree with it. It does not contradict what I said. When it says God cannot, that does not deny that it is impossible for him not to lie, by virtue of his very being what (or actually, I say who) he is.
To try to show what I mean: God does not do good because he decided that good is a good thing to do or be, but good is what it is, because God is good. God did not create himself --he simply is. He does not decide to be what he is --he simply is. You could say he cannot be otherwise, and that is true, but not because he chose to be true and consistent, but because he is.
This is how, or why, he is the only truly sovereign. We say, for our own understanding, that he is bound by his nature, and that's ok, but it is much more simple than that. He does because he is.