No, that is incorrect. The usage of implementation vs. api is related to the classes that are visible on the compile classpath vs. the runtime classpath. If the A library has an implementation dependency on B, this just means that the app can’t compile against classes in B directly, but B still must be loaded in order to not have a ClassNotFoundException when using the parts of A that use B with typical usage.
However, if you have a case where you know that the app will never need to use the specific features in A that have the dependency on B, you can exclude B. This is an independent decision though. It doesn’t have anything to do with using implementation.