The link also contains the answer to this question.
It sounds wrong that securityIdentityImpl is the right stuff - having "impl" in the names is wrong IMO.
I'll ping CDI guys.
For me, having "impl" in the name is wrong too but JBT works correctly here. You have to use <security:IdentityImpl> instead of <security:Identity> to have it working. Identity is an interface and can't be configured as a managed bean in seam-config.xml. Looks like an issue of Seam Security documentation.
And BTW we are going to rephrase the problem message for such a problem to make it more clear than "Cannot resolve member in node security:authenticatorClass".
Yeah, seem to all be a config doc issue - validating tools FTW