No, you're not basing your assumptions on how genuine someone is being, you're basing your assumptions on your assumptions of how genuine they're being.
Dismissing someone's arguments by establishing they're acting in bad faith is a valid rhetorical tactic but it doesn't work if you can't establish that. And labeling their argument as something it isn't doesn't help with that.
Addressing someone's presumed ignorance is helpful because you're also providing information for onlookers, pointing out the harmful effects of what someone's saying (like potentially muddying the waters when it comes to recognizing dog whistles) is constructive, attacking anyone who may be acting in bad faith but could just as easily just be ignorant is just a waste of your energy.
I'm not particularly interested in defending the person you replied to, I don't think they made a good case either. I just want people to be a bit more discerning with the terms they use. (And I have a compulsive need to correct people which I'm aware is really annoying.)