Are they right? Do we act for selfish motives alone?

Doesn't this entirely depend on how you define "selfish"? It's not clear to me why you set up these two hypotheses as the difference between egoists and altruists, as one could easily explain the empathy-altruism hypothesis in the language of selfishness.

Indeed, I would take the experiment whose results you quote as telling evidence that human altruism is selfish, powered by the degree to which we recognize ourselves in others. Yvain seems to strike at the heart of the issue much more closely by talking about 'reinforcement' rather than ascribing political names to desires.