Counterpoint: Tau were added well after GW had largely stopped with the topical political references, and were strongly unironically positive in their earliest appearances. The addition of gets-hot railrifles was a notably darker element in Tau at the time and was still lighter than the IoM.
Darker elements have been added, but the faction functioned without that element in their original form which has changed far less than other factions’ (due to the relative newness of the faction) so it’s not core to the faction’s identity.
Additionally, virtually all of the early dark elements in the Tau were either commonplace in the IoM or significant improvements over the IoM’s status quo. Through your suggested lens, this would have some interesting implications.
Less social mobility than the people with hereditary nobility? It’s not great, but the IoM are pretty poor at it. I suspect you misunderstand the T’au caste system; admittedly it probably shouldn’t be called that as it doesn’t really function like a real world caste system.