Couple of common fallacies here:
Morality to an atheist worldview is a relative thing. It is based on preference and behaviorism. You can't get an ought from an is, a prescriptive from a descriptive. You can describe what you like (subjectivism/behaviorism) but that doesn't make it good, and the problem with relativism is that no society or culture can be any better than any other. If you hold a materialistic worldview then truth and values are measured through the five senses. How can you measure goodness through those senses (the descriptive)? Values can't be measured by the same token
Repugnant Hitler's Germany is no more wrong than Kim Jong-un's North Korea or Trump's USA.
I don't see anywhere that an atheists must believe or accept relative morality. The only necessarily limitation is that, whatever kind of morality they believe in, it can't be sourced to a god.
Furthermore, I don't really see how theists get a pass here with respect to morality. Different religions, different denominations, carve morality out in different ways. The morality one ends up with is almost entirely based on the random circumstances of their birth and upbringing, which are entirely relative and subjective. The only difference is, religious morality has the lack of humility to describe itself as objectively right and everything else as objectively wrong.
Given this, the objections levied against "atheist" relative morality aren't unique to it. It is correct that, without some higher level framework, you can't judge between different relative frameworks, but again, that applies to the variety of different religious frameworks as well. How can we say that Christian Morality according to the Bible is "any better" than Islamic Morality according to the Quran?
The most egregious error is this notion that because a system is relative, it can't be used to judge anything. Well that's simply false. The entire point of a framework is to make such judgments. It's just that different frameworks can judge the same situation differently. It doesn't eliminate the ability to judge one as better than the other, it just means that such judgement is only relevant to those people that participate within that framework. Which, for yet another time, applies just as much to religious as it does to irreligious frameworks. You think a Buddhist gives a shit as to whether or they're observing the Sabbath?