Unfortunately they have reached the logically correct conclusion based on their stated worldview. In the (paraphrased) words of the late Francis Schaeffer,
Without a transcendent moral authority, one that stands outside of our own human experience, then all of morality is relative and what is right or wrong will ultimately be determined by who has the greatest ability to force their definitions of right and wrong on everyone else
It is a sad state of affairs, but if we can not even agree on what the authority is for defining right and wrong we can't even talk with each other about it anymore.
Can you and I agree that the basis for defining right and wrong is set forth in the 10 Commandments and the Judeo-Christian moral code? If we can, then we have moral absolutism, not moral relativism.
No, they're not. They simply moved logically from a flawed premise (no absolute right and wrong) to an illogical conclusion.