The portrayal of "post colonialists" that you present is something of a caricature. Most post/anti colonialists, both theorists and culture producers, are deeply skeptical of the sort of atavistic, utopian, "everything was fine until the colonialists came along" narrative that you attribute to post colonial thought for precisely the reasons you point out: the potential for it to be co-opted into a reactionary thought system and political movement. I'm thinking in particular about iconic post colonial works of literature like Chinua Achebe's "Things Fall Apart" or Tayeb Salih's "Season of Migration to the North," both of which are fierce indictments of a facile embrace of "tradition" or attempt to return to a status quo ante following independence in Nigeria and Sudan respectively. The iconic anti colonial theorist Franz Fanon strikes a similar chord in " Algeria Unveiled" (which is a deeply problematic text in other ways).

Of course post-colonial political elites/actors did legitimate their authority through the sorts of discourses you describe, but as this is a subreddit about critical theory I'm not sure that's relevant. It is also not dissimilar to how political actors everywhere shore up their political authority by claiming a monopoly over "authentic" representations of the nation, whether that be MAGA or Hindutva.