There are two types of speculative knowledge: knowledge of facts and knowledge of value.
Statements of value (both ethical and aesthetic) do not appear to be empirically verifiable.
Showing that all knowledge of facts must be empirically verifiable does not show that all synthetic statements must be empirically verifiable.
So radical empiricism must be wrong.
Statements of value, insofar as they are significant, are verifiable in the same way that all synthetic statements are. Insofar as statements of value are not scientific they are not in the literal sense significant but are simply expressions of emotion that can be neither true nor false.
This classification into significant and not significant applies to statements of aesthetics as well.