It would be better of the poster clarified what they mean by 'incapable'. Incapable by figurative sense or literal senses. Well seeing as how stupid is used as an insult it wouldn't fit.
As I see it from y studies in neurology and archetypal psychology, there is no such thing as 'incapable' of thinking unless the brain is fully dead. Even brain dead people have thoughts just not conscious ones or they are unable to communicate them. Thus unless the brain is fully dead there is thought. As long as the brain is functioning, even if on levels that are undetectable there is thought even if it's just the brain telling the body to breathe.
However even those with anencepholy (such as this one boy I heard about) had such condition yet grew to be basically fully functional despite only having a brain stem. So is it really a matter of the brain or the body? The body can think too. It's just up to the brain to decode these messages, unfortunately they often go dismissed or outright unrecognized by the conscious mind.
By these standards, every single cell is alive, every single cell can feel, experience, and hence think.
As long as there is neuron activity there is thought on a brain respect. As long as any organism, multi celled or single celled is alive there is thought. As well as thought that 'has no meaning or doesn't make sense'. How the brain actually works is what we are really talking about here. Our bodies will often warn us about deficiencies and such, but the conscious mind will interpret that as 'jumbled nonsense' but we have the thoughts because our inner minds know what is going on.
Even thoughts that don't make sense to what we interpret at a conscious level does not mean it is 'incapable' it's just impossible to understand at that level with our conscious understanding.