I don't think it's possible for apes who evolved and outcompeted other apes in the endeavor to become the dominant species on a planet to be truly altruistic. At least not without a lot more cultural, social, and possibly biological evolution, if ever.
Doesn't matter though. What difference does it make how "true" one's altruism is?
Doing good for others should feel good! And if that internal reaction helps us want to continue doing good for others, then that's also a good thing.
I think also we should be less concerned about labeling ourselves this way. I would not try to make "altruist" a part of my identity (or "egoist", for that matter). You are just someone who does good things for others, and -- incidentally -- that makes you feel good. That should be enough.