It seems to me that what this comes down to is: is it justifiable and appropriate to categorise a person according to their behaviour (including the holding of opinions)? The author doesn't seem to deny that people can exhibit stupid behaviour, but argues that its wrong to categorise a person who exhibits such behaviour because you will (inevitably) never have enough information about the person or situation to make a judgement about the intrinsic qualities of the person. This might be true in some theoretical sense, but it doesn't really sit well with human nature or needs. We all have finite lives and stuff to be done in those lives. Making judgments about people is necessary to avoid a frustrating life spent (as in 'used up') dealing with people who behave stupidly.
And really, how long do you have to evaluate someone's behaviour before you are no longer 'jumping to conclusions' about them? Strangely, the author doesn't say. At some point you have use the history of a person's behaviour to decide[1]. A purist might say that such a decision should always be tentative, but in reality I think they soon become fixed. We are talking about people here.
For what its worth, my approach is that if I decide to think of someone as being stupid (or smart) then I try to be conscious and mindful of the decision, allow myself the opportunity to change my mind (while recognising that I probably won't), and always respect their dignity as a person.
[1] I suspect that how soon this happens will mainly depend on the social relationship between the people involved.