You are correct that there are a variety of ways to implementate non-repudiation. Typically these require a third party. I should have been more specific about the distinction I was attempting to describe.
Public key based signatures are traceable to a single key holder.
Symmetric techniques are subject to repudiation since both the creator and the validating agent have access to the secret key. The semantics of a public key signature are different than that of a keyed hash. Our specification should clearly describe the diffierences in service provided by the two mechanisms. The syntax of our XML digital signature should clearly describe the expected security service. We should not confuse keyed hashs with public key based digital signatures.