Ignoring dhh because he's offensive is totally reasonable and if that's what you meant well that's different.
It's fully based on the substance of the point as well. DHH's view is not just wrong, it's wilfully misleading, on a topic of enormous significance, to further a bad faith point. He could have retracted or amended it when presented with the facts; he has not. So this is a problem of substance as well as person.
From there yes, I get to a point about the person. IMO it is a useful practical definition of madness to pretend that someone acting in bad faith can be trusted to do it only on a single divisive topic.
Given two people, one a manipulative antivaxxer and one not: are you saying you don't factor your view on their anti-vaxx thoughts into their recommendations on other fact-based topics?
For me, these people simply get deleted from my brain, as far as possible.
These are simple judgements about the likelihood someone's opinion on a topic I do not know about is going to be good. When I know that their other opinions are clearly wrong and held in bad faith, I am going to take that into account.
(Where religion comes into things is where it becomes mostly a question of personality.)
Manipulative is a judgement on a person. That aside, let's say the person is a NFL quarterback. No, their stance on immunology would not play a role in their authority on NFL strategy... you know, the domain they are an expert in.
The inventor of the MRI was a young earth creationist. Perhaps a rare oddity, but just an example of how someone who works in science can still be passionately wrong about something despite evidence to the contrary.