It is an example of intellectual diversity even though nazis are monsters but I think the overarching point of the article is that despite the fact that monsters exist intellectual diversity is good.
If the author wants to argue that we have less intellectual diversity now and its bad, finding some examples of ideologies that don't want to mass murder specific ethnic groups would go a long way to making the argument convincing.
[And i'm starting from a place where i believe intellectual diversity is a good thing, although i think we have more now than we did in the past. Its not like i disagree with the author on that point, I just don't think s/he actually cares about intellectual diversity and instead just likes nazis. ]
For example, Edward Herman and Noam Chomsky talk at length about how the way our media is interlinked with corporations and governments steers the discourse toward a narrow range of ideas approved by those groups in their book "Manufacturing Consent". Mark Fisher decries a climate in which most people consider markets as the only possible solution for all problems in "Capitalist Realism".
However, because these authors can just be upfront about what their beliefs are, they aren't forced to argue for vague concepts like "intellectual diversity" and can instead be specific about the type of restrictions that are occurring and in the case of Herman and Chomsky even track them with real numbers, making for a much better argument.
(Cue the "I don't support nazis I just think that for the good of mankind, unfortunately, we need to do everything we can to support nazis, incidentally of course" script.)