—“Do you believe that morality beyond self-interest is entirely false as a result?”—
I don’t believe in anything, because the term is archaic. I can state that it’s a strong truth candidate, because despite extremely exhaustive efforts by highly biased researchers, we cannot find a single instance of moral action that is not in itself selfish through kin selection.
Now, when we use the word ‘moral’ we must grasp that there is an objective morality in natural (necessary, consistent, and decidable), and normative morality (local group contracts for different sets of behaviors that produce group benefits from which individuals largely benefit), and individual morality (those subsets of moral choices I choose to follow and not). We conflate these two terms, just as we conflate law (natural law), legislation (contract or command), and regulation (arbitrary edict). But objective and normative, and individual morality are equivalent to natural law (true), legislation (contractual), and regulation (arbitrary choice).
When I write I use moral for objective morality of natural law, and norm for normative morality of local normative contract.
We can extend this basic principle from not only sentient cooperative groups, but to non-sentient groups, to non sentient individuals, to plants, to bacteria, to the natural elements that make up the physical world, and to our emerging understanding of the physical world: that we must fight entropy if we wish to survive.
So it is not only illogical to engage in self-destructive action, but it is physically impossible so to speak, as it would violate physical laws of the universe.
Now some creatures appear to do sacrificial things, but this is sacrificial only from the (fallacious) human perspective as individual pleasure-seekers. But from evolution and the physical world’s standpoint, once we have exhausted a BENEFICIAL reproductive role we are no longer valuable to the organism (the algorithm) as a whole. Thankfully humans are almost always beneficial to one another when they are alive and not harming one another. Even then, those who harm, may be benefitting the organism (algorithm) “man”.
Now when we say self-interest, selfishness that signals possible parasitism, or non-payment for commons is something all creatures that cooperate retaliate against. So there is a difference between COMPROMISE (rational self-interest) and ABSOLUTE (and therefore irrational) self-interest. What is rational for all of us is to preserve the incentive to cooperate, and to prevent providing incentive to retaliate, yet being defensive enough to discourage offense against us.
So in this sense, it is always rational to compromise with those with whom you are compatible, because compromise with those with whom you are compatible is in your self-interest.
There are no rules without limits. If we cannot state the limits of any general rule, we state a falsehood because we cannot state a truth. This is why the wise speak in teleological ethics (science/outcomes), the informed but inexperienced and deceitful speak in deontological ethics ( rationalism/rules ), the young, lacking knowlege and experience in virtues (analogy/imitations), and children in punishments and rewards (goods and bads).
I hope this provided the answer you sought.
The Philosophy or Aristocracy
The Propertarian Institute