Nearly every religion preaches to be giving and kind to those in need. It's absolutely not a non-sequitor to admit that a large number of atheists don't believe there is any guiding morality to the universe and that we have to come to our own conclusions about morals and ethics. Moral relativism is a generally accepted thing among many atheists. This does not mean all atheists are selfish, I would classify most as Humanists. Rand was mostly an outlier.

She was able to promote the idea that selfishness could be good because she didn't ascribe to any religion that defined that as a sin.