The belief that “all cultures are equal” is a necessary corollary of the belief that all inequality in outcome is virtually always the result of oppression.  But claiming “all cultures are equal” makes as much sense as claiming “all things are equal,” implying clouds are equal to rocks without specifying what they are equal with respect to, making the statement nonsensical.  Surely some cultures are more connected to, and more likely to create the conditions for achieving, certain goals than others, as in society with culture A prioritizes goal A and is more likely to achieve it, while society with culture B prioritizes goal B and is more likely to achieve that.  For example, society A with culture A may prioritize a life of being consistent with Allah’s teachings, while society B with culture B may prioritize the physical health and longevity of its populace, so it would be unsurprising if society A is more successful with its goal while society B is more successful with B’s goal. 

A complicating factor is when society C prioritizes war-making, and through that is able to dominate society A or B, and many can see that society C is not superior in achieving the goals of A or B though it forces its culture C on A and B, and so they rightfully conclude that C’s culture is not superior to that of A or B.  But that does not mean that “all cultures are equal” or that some other culture D of society D does not have the ability to demonstrate its success in achieving D’s goals that are more appealing to more people in all the societies combined than the goals of any other society with any other culture.

Also note that the idea that all cultures are equal may be promoted in a misguided attempt to show respect for and provide dignity to those who come from other than the dominant culture, though it only serves to further enfeeble them.  The culture that a group of people develops in a particular environment is an adaptation to that environment and as such has been molded by the forces present to allow the individuals to effectively operate in that environment.  Those from a different environment who have been shaped by a different culture will likely not be as effective in the new environment unless they adapt to its culture.  To encourage them not to adapt to the culture or even to advise them to mix in equal proportions their prior culture with the culture of this new environment is doing them a disservice.

The exception to this is when the newcomers from the different culture are able to achieve dominance through force or technological superiority.  Then they may provide pressure for structures and patterns in the culture of their new home to conform to their alien culture, and individuals in their new home may be required to adapt to the alien culture.  Also, note that some cultures have relatively more universal attributes that empower the individuals who absorb them to dominate others in other environments (e.g., a preference for the scientific method), so these cultures more often become expansive.

On another related point, it is often said that “Politics is downstream from culture.”  While not disputing that, I think a more complete representation would be an Escher-like endless loop constructed as follows:
(1) Politics is downstream from culture;
(2) Culture is downstream from both technological innovation and law;
(3) Technological innovation and law are downstream from economic policies;
(4) Economic policies are downstream from politics. (Loop back to (1))