Why are intensive properties not additive?
Its because intensive properties like temperature, refractive index, density are same for all the portions of the container of gas. You can't say that if the temperature of one portion is x and and other portion is also x, then the total temperature is $2x$.

But extensive properties, they are different for different portions of the container like mass, volume, length. So they can be added.

It doesn't matter if the intensive property is a ratio of two extensive properties. If two extensive properties are in ratio then if first extensive property changes then the second one should change.