Acts of charity, generosity, and volunteerism are correlated with activation in the brain’s reward centers (ventral striatum, medial prefrontal cortex). Helping others feels good, biologically. The altruist experiences hormonal reinforcement through dopamine and oxytocin — demonstrating that “good deeds” literally reward the doer. — Copernicus

Selfish people no doubt experience the same reward when they perform acts of greed and meanness and bullying. The difference is not in the hormonal reward, but in what acts stimulate the hormonal release. By focusing on the same reward that follows altruistic and selfish acts, you eliminate the distinction. Clearly, to you, the distinction is not important. Fair enough. But you can't prevent other people finding the distinction important.
No doubt people who harm themselves (cutting themselves, starving themselves) experience some sort of hormonal reward. You would no doubt call those acts of self-interest in the same way and ignore all the reasons why such actions are problematic and fail to understand why other people want to help, not merely observe. Addicts perform actions that are similarly harmful to themselves, and experience a certain reward. For the rest of us, it is not about the reward, but what stimulates the reward.
Your way of looking at these actions does not enable you to see such actions as problematic. That's your prerogative. Other people see things differently, and they are entitled to their view even if you cannot understand it.

I don’t deny that we are motivated to achieve k personal satisfaction, emotional fulfillment and meaning. — Joshs

Yes, but I think it is important to add that the differences at stake here are not about those rewards as such. They are about what gives us personal satisfaction, emotional fulfilment and meaning. People find those things in different ways, and that is where the moral questions arise.

When I perform an active of ‘selfless’ altruism or generosity, it is made possible by my ability to expand the boundaries of my self, — Joshs

In a sense you are right, of course. But that way of putting it doesn't distinguish what's going on from individualistic self-interest. It's more complicated than that. When I empathize or sympathize with someone else's predicament, I do not lose sight of the fact that it is not me that is sleeping in the streets.

Fair enough. — Ludwig V

Yes.

The more narrowly we are defining "selfless", the less importance the claim that selfless acts don't exist has.

That's on top of the fact, as already pointed out, that the conception of "selfless" as literally meaning having no concern for the self whatsoever, is nowhere related to what the word actually means.
(NB: I would guess some dictionaries might give a very terse definition, that implies no concern for the self, but they would also probably define words like "monopoly" or "democracy" in similar simple terms that would imply they don't exist either, if taken literally)

So if you want to create a term that means a willful action that's not willed, and not even originating in biology, possibly even causality...then sure, that doesn't seem to exist (or even make coherent sense). I'm with you on that.
Meanwhile back in the real world, people can be motivated by a desire to help others, putting their own needs second (within reason), and that's what people actually mean by the term selfless.

Meanwhile back in the real world — Mijin

...people call mass "weight".