Earlier quoted context omitted.
The saying presents it as a dichotomy, which is why the saying is wrong. It is also parroted way too often on HN. It might as well be, "Never attribute to hunger that which can be explained by incompetence." Is it easier to see why that fails as any sort of useful proverb?
It is not a dichotomy, as the saying doesn't say "it must be either malice or stupidity" - it only proposes to also keep supidity in mind in case malice is assumed. Nobody is saying there can't be a third cause - you're fighting a strawman. Besides, what third reason would be relevant in this case, exactly?
It doesn't need to. The false dichotomy is presented in the phrasing.
>what third reason would be relevant in this case, exactly?
It's not a third reason, which is why you are confused. It is a third possibility, not a third reason. That third possibility is that both malice and stupidity are present. In such a case, if stupidity could adequately explain the action, the saying suggests ruling out malice, which would be incorrect. That is precisely why it is a false dichotomy.
It's not a strawman, you just haven't thought about it correctly.