Can APM replace a WAM like TAM or SiteMinder?

If by "WAM" you mean "Web Access Management", then I'll start with a quote from the Wikipedia page on the subject:

Web access management (WAM) is a form of identity management that controls access to web resources, providing authentication management, policy-based authorizations, audit and reporting services (optional) and single sign-on convenience.

In this sense, APM does meet the requirements of a WAM. It provides FULL PROXY authentication services (separate client side and server side authentication functions), dynamic and policy-based authorization, robust reporting, and single sign-on. The most significant difference is that APM is an authentication proxy and not specifically an identity management product. Other than the local user database in 11.4, APM is not a user directory and does not store authn/authz information. Where a given WAM vendor typically focuses on a small set of technologies, APM can query and interoperate with many different directory and IdM products and technologies, and is typically deployed to provide dynamic access control, single sign-on, and secure pre-authentication services (among many other capabilities).

To answer your second question though, there are areas and situations where APM can be complementary to a WAM, or in other cases completely replace it.