I meant that they do not have the same natural inclination to learn the subtle nuances and mechanics of language that J's do, but they can develop it if it's needed of them (and probably will to some degree in school). Instead, they tend to focus more on understanding reality itself, in a way that isn't linked to langauge. This makes them considerably more intelligent in a non-verbal way, but the price of that is that they're less likely to acquire the skills needed to express this understanding, unless they manage to find the motivation to do so.
So, INTP's (like other P's) can be skilled with language if they work at it, but they aren't naturally inclined to be skilled with language as most J's are.
It's roughly the same idea as J's being left-brained, and P's being right-brained.