That would be an incomplete example. As you guess you can use both ser/estar.
It is not very good
If we refer to, for example a deal, we would use
(el trato) no es muy bueno
If we refer to the condition of something, we would use
¿Cómo está el coche? (meaning what is the condition of the car?)
No está muy bien.
¿Cómo está la cena? (meaning how's dinner tonight?)
(No) está muy buena.
And then you have also a problem because
good can be translated both as
bueno. So "it is (not) very good" could be translated as
- (no) es muy bueno
- (no) está muy bien
- (no) está muy bueno
So basically I think that there could be a lot of options and example did not cover all cases (probably I didn't either in my explanation...). It is not that you can only apply one verb for positive or negative, it is just that the example is not exhaustive.