Atheistic Existentialists argue that there is no inherent human nature, and that what we call 'human nature' is actually the product of a human being's decisions and actions, not the cause. Therefore we cannot be 'inherently' selfish, we can only act selfishly.
Human attitudes and behavior are constantly reshaped by the changing economic systems in which people find themselves. There is no such thing as universal "human nature" and nothing that points to there being one.
Taking this is as true, an act's outcome is still not necessarily the cause of that action. Example: a man benefits from giving to charity because there is less poverty and therefore less violent crime. He may have given to charity altruistically (or for any reason). The outcome was not the cause.
Self-sacrifice isn't automatically altruistic. Falling on a grenade that a comrade might live might still be a selfish action, if you consider the virtue of bravery, and the dedication to whichever cause unites one with one's comrades as components of one's identity.
If an individual's preference is to be altruistic, they gain intangibly from acting altruistically. Therefore it is impossible to distinguish between altruism 'for' the actor or 'for' the recipient, as both benefit, and both occur simultaneously.
Doing things for others at one's own immediate disadvantage is likely to lead to an advantage eventually, either directly of indirectly, to the one being altruistic. Altruism can be to one's own benefit.