It's an issue, insulting a group is always in one way or another insulting anyone who believes they are part of that group, it's stereotyping, and the cause of tribalism, racism, sexism, etc. However at the same time, one can use generalities and also be mindful of the fact that not everyone under that generalization belongs in that group.
For instance, River claims that you or Bones have said that if you're a Stotts supporter that you don't know basketball, do you believe that to be one hundred percent the case? Obviously Dame thinks he knows basketball, he was on Rick Carlisle's staff, he's played professionally. I'm guessing you didn't mean Dame, in particular, doesn't know basketball or Carlisle doesn't know basketball or a plethora of other NBA guys who are Stott's "supporters".
My point is generalizations are definitely dangerous and can lead to weird arguments like this, but we all use them to some extent and we all know that they don't apply to everyone in a particular segment of the population who identify a certain way.
What Marang did was try to belittle all Anti-Stotts people by calling them basically an idiot, and he was wrong and his argument weak, but it goes both ways, and we all need to be mindful of that when using generalities, and calling out every single person for their use of generalities is going to be pretty time consuming.