Solving an atheist's dilemma?

Given my interest in the moral dimension of the debates between Christopher Hitchens and Dinesh D'Souza (and Alister McGrath), I was delighted to stumble across an attempted solution to the atheist's ethical problem at "Daylight Atheism" in a post apparently by Adam/ebonmuse.

I'm not going to reproduce the entire thing, as the argument itself should be fairly compact.

You worried that atheists have no compelling answer to a person who says, "I'm going to do whatever I please." But religion does not solve that problem. If anything, the problem is far worse ...
(Daylight Atheism)

Adam's argument is presented as a response to an editorial by Michael Gerson, the "You" in the first line. This isn't likely to be the argument, but if it were then it is nothing more than a tu quoque fallacy, the technique of excusing one failing by taking note of the same or a similar failing on the part of the opponent. No need to linger on this, I trust.

Unfortunately the gist of Adam's argument apparently occurs in the following slippery (in the mercurial sense) paragraph.

You asked what reason an atheist can give to be moral, so allow me to offer an answer. You correctly pointed out that neither our instincts nor our self-interest can completely suffice, but there is another possibility you've overlooked. Call it what you will - empathy, compassion, conscience, lovingkindness - but the deepest and truest expression of that state is the one that wishes everyone else to share in it. A happiness that is predicated on the unhappiness of others - a mentality of "I win, you lose" - is a mean and petty form of happiness, one hardly worthy of the name at all. On the contrary, the highest, purest and most lasting form of happiness is the one which we can only bring about in ourselves by cultivating it in others. The recognition of this truth gives us a fulcrum upon which we can build a consistent, objective theory of human morality. Acts that contribute to the sum total of human happiness in this way are right, while those that have the opposite effect are wrong. A wealth of moral guidelines can be derived from this basic, rational principle.

This answer fell well short of my extremely well tempered expectations.

On what basis does Adam distinguish between "instinct" and "empathy, compassion, conscience, lovingkindness - but the deepest and truest expression of that state is the one that wishes everyone else to share in it"? By all appearances, Adam quietly concedes the argument just before loudly declaring victory in contradiction to his earlier statement.

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