Again, the emphasis is on the "can". INTJs can also be narrow-lived, step-counting trolls whose in-born intelligence is rivaled only by their bigotry and defensive, hasty conclusions.
That said, they can also be some of the most intellectual people around, surpassing even INTPs, who, it must be said, have the greatest inclination toward intellectual pursuits.
This is because: the intellect, like the physical body, is something that must be fostered and attended to. The analogues between physical and mental training are striking.
For whatever reason, an uncommon degree of drive attends the introverted intuitive's mindset. This drive will often lead INTJs to make the long-term effort required to develop their minds and rational abilities.
As I've stated, the INTP mindset is, if descriptions are correct, unquestionably the most inclined toward this development. But just as the SP mindset, which places kinaesthetic intelligence at the forefront of the psychological processes, inclines members of that group toward prowess in physical pursuits, this inclination must be acted upon and fostered for it to bear fruit.