There seems to be a muddling of concepts in the article. My understanding of the concept of core grammar comes from Steven Pinker. The core grammar is not a syntax tree ready to parse anything, but a set of instincts that can be applied when learning you native language. So you can learn word order syntax or tonal syntax; one of a small set of patterns. However if you have not learned the the specific rules for a given language, you will not be able to recognize them. Recognition of the passive may be in there, but you will not recognize if you havent learned it.
Failure to recognize "every" as the universal quantifier is silly. In normal conversation "every" is not a universal quantifier. It signifies a general rule, something quite different. One can say, "every dog has 4 legs", but not be thrown into logical confusion by the sight of a dog with three legs.
Ironically, I am unclear on whether their use of "cannot" and "unable" means that some people could not, even in principle, understand these rules of grammar, or just that there are people who currently do not understand them, but could be taught.
(Meta-ironically I am now wondering if the perceived lack of clarity is really revealing something about myself. :) )
Failure to recognize "every" as the universal quantifier is silly. In normal conversation "every" is not a universal quantifier. It signifies a general rule, something quite different. One can say, "every dog has 4 legs", but not be thrown into logical confusion by the sight of a dog with three legs.