You need to say, "As far as I can tell," or at least, "As far as I and the people I hang around with can tell." But to speak as if one had comprehensive and universal experience is often a perilous exercise, logically. You will find that others think differently than you say; we are not all secret Atheists.
I understand that exactly. But if true, it turns the OP into nonsense. Because the OP assumes there is something subjective called "morality" that could potentially, if proof is forthcoming, justify the reunderstanding of morality as "objective": it asks, "What could MAKE morality objective?"Not just for morality, but for everything else too. That's where we have to reinvent the whole world anyway, including morality, as a quasi-objective something, but it has no genuine foundations anymore.
Maybe that was an insincere question, on Peter's part. Maybe he meant all along simply to say, "There is no such thing as morality, so there's nothing that even potentially could be made objective," but he wanted us to think he was open to evidence. But I took him as at least a little sincere. I supposed he was open to counter-evidence, and to being convinced if such could be found. But I suspect now his disbelief was always unfalsifiable, because it was presumptive -- that is, it preceded as an inevitable byproduct of having believed, without proof of course, that Atheism is just true. After that, Nihilism was inevitable, just as you say.
That's what the late Existentialists thought...you know, Sartre, Camus. It's not what the first Existentialist, Kierkegaard, thought. Kierkegaard agreed we're "thrown into" the world, and come into it without our agreement, just as you say...but he interpreted that not, as Sartre and Camus, that one simply had to "make choices" with no information, but rather that it is a basic fact that reminds us we are contingent beings, not the centre of the universe.That's just how the world is, I was born into it just as much as you were, without being asked whether or not I agree to its terms and conditions. One can learn to deal with it, or seek refuge in delusions.
This world existed before you and I arrived. Presumably, it will exist after we are gone. Even Sartre and Camus thought that was true. So we are not self-sufficient beings, not eternal, and not necessary; we are here by the grace of God. And Kierkegaard pointed out that that opened up not only the space for faith but the necessity of faith. Because you and I are awfully small and local; and if we do not invest ourselves in the right way, placing faith in that which is the true Centre of the universe, the true Reason and Cause that we are here, we will not do well. Our lives will not become authentic, and we will not achieve the telos or purpose for which we found ourselves '"thrown into" life.
What you take as a rationale for total Nihilism, Kierkegaard saw as a fundamental necessitation of faith.