The passage suggests that not all na-type adjectives can be used as real nouns as shown in (b). It doesn't seem to be a case of careless choice of words, since in the 2nd para, the passage uses "some" and in the 3rd para, the passage uses "all", and there is an explicit contrast.
I was wondering why is it that not all na-type adjectives can be used as real nouns. As far as I know, all na-type adjectives can be used as if they are real nouns, like for example, this is grammatical right:
特有は確かだ
Is there even 1 na-type adjective that can't be used as a real noun as shown in (b) (to prove that passage right) ?
特有は確かだ
sounds ungrammatical.