c'est used even though is expressing opinion of a known thing (and known gender)

Question

"Cette pomme est bonne. - Oui, c'est bon.". Why is "c'est bon" used here instead of "elle est bonne", when it's expressing opinion over something specific that you know the gender of already?

Answer

That's always a tricky thing to wrap your head around. The rule that general statements require ce and specific ones il/elle, is a good guideline but not a hard and fast law. Here's my attempt at an explanation:

Cette pomme est bonne. - Oui, c'est bon. Elle est vraiment très juteuse.

The first part of the reply is an idiomatic c'est bon -- it's good. The second one is a characterizing statement about that apple in particular. For that you'd use elle.