Interpreting the statement charitably, I think they mean "morality and ethics are arbitrary constructs of humankind, not the deep universal principles they are sometimes taken to be".
I would love to hear what you think. I have a strange mind and I'm just trying to understand and learn.
If I am against something I can say "I am against X"; making statements about oneself is generally safe and not an overstep. There's no obvious path from "I am against X" to "I will stop you doing X".
If I say "X is wrong", that sounds like an absolute, and is more easily (ab)used as a justification for stopping others doing it.
The exact same linguistic distinction applies to offence. "I am offended by X" is accurate and works fine in a society. "X is offensive" is a precursor to censorship.