In my opinion no healthy INTP can be really below average because his brain is especially tuned to achieve certain tasks (logic, understanding, problem-shooting, each governed by Ti) and through prolonged use, (assuming that a prolonged and regular use occurs) this ability gets more and more refined and well, 'intelligent'. It's a simple matter of preference. An ESFJ, at the same time, maybe will not have this right kind of intelligence as natural as we do, but will be much smarter to things INTPs would have no clue of. A bright example is social life, which requires the said 'interpersonal' intelligence.
What I'm trying to say, is that every healthy human being is highly intelligent (and gets more and more so) in some reasoning process, simply by 'training'. Which skill will be trained the most, is determined by preference - as established by MBTI. Therefore it is paradoxal to think of an INTP, whose brain is by definition tuned onto logic, who has no clue about how logic works. This should imply either a damaged or misfunctioning brain, and as we're talking of healthy people, this is necessairily untrue.
As 'logical' intelligence, which we are treating of, lives in the region of the brain INTPs are used to, an INTP scoring below average on 'logical' intelligence would have this region underdeveloped, as we are supposing that that region is regularly and naturally stimulated and improved up to its task.
Otherwise a less frequent cerebral activity occurs.
It has been seen that INTJs and INTPs tend to score highest on IQ tests. That is simply because THAT is their realm of intelligence. But try to imagine an INTJ or INTP, both commonly stereotyped as cold, taking active natural interest in other people's feelings in order to improve their welfare. It'd be rather a skill to learn to them, not just a natural and, especially, energising process as for ESFJs.
This is because every human being masters his preference. If they can't master it, there must be some 'disease'. So we have all healthy INTPs average or above average 'INTP' intelligent and all healthy ESFJs average or above average 'ESFJ' intelligent.
Therefore, intelligence meant in its classical meaning (the logical one), the meaning, that is, which was probably meant in the article, every INTP is average or even likely to be highly capable of. Therefore, if the premises of uselessness are below average intelligence, No INTP is Useless, Quod Erat Demonstrandum.
Any thoughts about this?
(keep in mind that I worked on simple intuitive truths about brain, functions, and brain development given that I'm ignorant of such a complex subject)
(and yes, I believe intelligence - better said, the 'kind' of intelligence - has correlation with personality)
(AND SORRY THIS WAS TOO LONG, but I'm so excited that I just demonstrated I am not useless)