That maxim seems far too cautious. It would, for instance, suggest that you can't have a "fruitful discussion" about installing wheelchair ramps with people who identify as disabled; nor can you have a "fruitful discussion" about neurodiversity with people who identify as autistic, ADHD, etc.; and so on.

Well, I don't understand mere conceptual proximity to imply engagement in Graham's sense. Neurodiversity for example implies a particular approach to neurological issues; you can identity as ADHD etc. without identifying as a neurodiverse individual or being identity-entangled with any particular attitude toward that model. If you are talking to someone who identifies as neurodiverse, though, or who's adopted an identity directly excluding that identity, then I think Graham's caution applies.

That's not to say that talking with such a person on such a topic is necessarily a waste of time, though; one or both of you might learn something about the arguments being used, or about facts to apply. And you might find your opinion shifting, if you don't have an identity in the game. What you can't expect is to shift an entangled opinion without first breaking down the identity it's entangled with -- and trying, or even being perceived as such, is a good way to send the discussion straight to hell.

I do agree that a presumption of bad faith would exclude fruitful discussion in a much stronger sense.