“Ultimately” is the key word here. It is because secondary causes are real that natural science is possible. When we study the physical world, we are studying how physical things themselves behave given their nature, not the capricious acts of God. And it is because immanent teleology is real that natural law is possible. When we study ethics, we are studying what is good for human beings given their nature, not capricious divine commands. Ultimately the facts studied by science and the facts studied by ethics depend on God, because everything depends, at every instant, on God. In that sense, science, ethics, and everything else depend on God. But proximately ethics can be done at least to a large extent without reference to God, just as natural science can. In that sense, many moral truths would still be true even if, per impossibile, there were no God – just as the periodic table of the elements would be what it is even if, per impossibile, there were no God. (All of this is discussed in chapter 5 of Aquinas. And see the first half of this article for a sketch of A-T natural law theory.)