Confusion with the term "selfish"

"Selfish" typically denotes the tendency to favour self-benefit over benefit to others, even at times to the detriment of others.

But what does it mean to be selfish when one believe the self is everywhere? Some monks which reach enlightenment claim to have no preference for family and friends over strangers because everyone is their relative, or made from the same entity or divine substance, or everyone is equally favoured. They believe in unity. Similarly with psychedelic experiences one can find themselves dissolved away into a un-confined state of non-ego or non-identity where everyone is one and the same, connected and fluid. That the self is essentially all awareness and that individualism is a convenient illusion of boundary/limit created by spatiotemporal nature of awareness (ie. Multiple perspectives) but not intrinsically based in physical laws of nature.

So if I were to embrace the concept of self extension to all beings. What does it mean to be selfish? To do things for the benefit of all things. Is there any differences here to selflessness?