Not true, I think. If a number of individuals share the same,
or at least similar, value-system, that morality implies
behavioural dynamics. Morality A may have game theoretically
stable points at "better" value-sets than Morality B. So even
though the value-sets may be higly subjective, the value-systems
are not.
The most obvious way a morality A can be better than morality B
is the amount of synergy it promotes.
/felix