In 2.4.3. Injecting custom policies with the 3scale operator,

To use the 3scale Admin Portal to add the custom policy to a product’s policy chain, you must also register the custom policy’s schema with a CustomPolicyDefinition custom resource. Custom policy registration is a requirement only when you want to use the Admin Portal to configure a product’s policy chain.

isn't related to following sentence. Using CustomPolicyDefinition is another way to add custom policies. Refer to the upstream document