Given that Yorkie referenced the original writers in the first place, it is fair that we discuss the original fluff- as the aforementioned writers intended it to be. Here is a screen grab from 'Ere We Go':




Where you can clearly see the word caste being used. Orks have been a caste based society in the minds of the authors right from the start.

In fact the system of tribe/clan/caste is directly analogous to the Tau system of septs and caste.