This post would be better if it did not use an absurd imaginary conversation to obfuscate whatever real world thing it is trying to describe. You can just use a real conversation.
It's been my experience that showing the real conversation directly encounters some emotional barrier in people who actually hold that belief, makes them defensive and less likely to see the point. This doesn't apply to most of LW audience, but still.
Alice and Bob are hanging out when the following happens:
Alice: I'm hungry, can you bring me the cake from the fridge?
Bob: Yeah one moment... Damn, I just checked and it looks like this cake is plastic. We can't eat this.
Alice: Oh, damn, that sucks. Do you have another idea?
Bob: I'll try to cook something.
10 minutes later
Alice: I'll be honest Bob, this food you just made is not good. Just give me the cake.
Bob: But even if my food is bad, the cake is still plastic.
Alice: I get it, but I'm hungry and I need something to eat and your food isn't enough, give me the cake.
I've seen an equivalent of this conversation go down around God and morality, essentially people suggesting that if atheists are failing to come up with satisfying morality, then God and God-given morality must be real. This fallacy is a version of appeal to consequences: it confuses what we would prefer to be true with what is true. Bob’s bad cooking tells us something about Bob’s cooking. It does not tell us whether the cake is edible.
To be clear, this post does not claim non-existence of God, and does not make a statement about objective morality either, merely that "atheists can't figure out morality, therefore God exists and God-given morality exists" is fallacious.