If they want to state that no nazi's see advertisements on their platform, they are free to make that claim. But I know there are nazi's on that platform and I know they get served ads. What else needs to be known?
If they weren’t interested in the content itself but were instead interested in studying or manipulating the system providing the content I can see why the behavior in question would be deemed inauthentic. However, I can’t really back it up beyond that as I’m not them, sorry.
Twitter's implenmentation of authentic user detection must be expected to try to separate normal users from e.g. test accounts created by client authors. And even if the Media Matters staff was genuinely interested in seeing etc, I think it's quite likely that their click pattern resembled a test account more than a Joe Normal User. (Intense activity immediately after signup, for example.) If that's the case, then Twitter's authentic user detection would classify the MM user as "likely test account" or something like that, and the authentic user data shown on Twitter's dashboards would exclude the MM user.
Nazism is bad but being a Nazi supporter or sympathizer doesn't make someone's usage inauthentic. At worst it makes their usage harmful.