The class of a building is just a shorthand way to describe its construction and/ or function. It’s easy to find the definitions by a Google search.
The class of building has no bearing on the cost of a repair, save that the BCA may require more stringent construction methods than original build.
The council can’t wave any such building requirements as it’s legislated at the state level.
If you think it’s too expensive, get your own quotes for comparison. Remember thst the strata manager is not all knowing.