I avoided diving into this before because this topic is about how atheist ethics can be grounded in objective fact, but again, you don’t have a proper understanding of how Catholic moral teaching is grounded. We are, largely, essentialists.
Take for example the idea of triangularity. It is objective fact that triangularity, being a triangle, means having three straight sides whose angles, in Euclidean space, add up to 180 degrees. You can change the words and the system of measurement, but that objective truth about that which we call triangularity just is what a triangle is. That is a real definition, not nominal. Likewise, there are certain true things that simply follow from being human, and that includes what is good for a human being. Two arms, two legs, two eyes, and yes, even proper ends of our behavior and the choices we make. This is not some artificially imposed design, but simply what being a triangle or being a human is from an essentialist standpoint. A triangle, and I don’t just mean the English word, but beyond that, the concept of a triangle, can’t simply be redefined as a shape with four straight sides whose angles add up to 360 degrees in Euclidean space. You change that fact and you are no longer considering what it is essentially to be a triangle, but what it is to essentially be a rectangle. Likewise, you change the ends of a human being, and you are no longer considering what it is to be essentially a human being but what it is to be essentially something else. The moral law is not something arbitrarily imposed on us, but just is part of what it is to essentially be a human. God had knowledge of this essence and chose to create it as opposed to some other essence. It’s not something that He can change, anymore than He can make a triangular-rectangle (or square-circle, as is more commonly said). It’s a contradiction.
There’s much more to be written, but Catholic morality is grounded in these truths that follow from an essentialist view of reality. This doesn’t mean we have total knowledge of what it is to essentially be a human or what have you, only that we are able to at least gain real knowledge of what it is to essentially be a human through observation and reasoning.
Now, we could certainly have a debate about essentialism, and also about nominalism, conceptualism, and realism, and furthermore between such things as Platonic Realism, Aristotlean Realism, and Scholastic (or Thomist) Realism (and maybe a few other branches), and perhaps further discussion about this essentialist view versus something like divine command theory, ways of thinking which might be unfamiliar to you, but that’s not what this topic is about, and I’ve only just scratched the surface.