Catalysis of the reaction: (2-keto-3-deoxynononic acid)n + H2O = (2-keto-3-deoxynononic acid)n-1 + 2-keto-3-deoxynononic acid. This reaction is the hydrolysis of a 2-keto-3-deoxynononic acid residue from a poly-2-keto-3-deoxynononic acid chain.