My text says that demonstratives (such as IS) do not agree
in 'Case' when acting as pronouns.
'Case' means modes of the declension such as Accusative or Dative
If the original noun the demonstrative is replacing is in the Accusative
then IS which is replacing it must be in the Accusative. You could not
use IS in the Dative if the original noun was in the Accusative.
Then why does the rule in the book say that they do not agree in case?