His (or any potential suspect’s) statement that the guy he shot attacked him would be considered self-serving unless it were to be corroborated by a neutral witness or evidence such as a condo-complex security camera. That’s just the way the law works.
I understand that his remarks that he was attacked works in his favor, and as such can be called self-serving - certainly remarks made after the incident. And assuming his testimony is "self serving" that way, it remains so, whether corroborated or contradicted by other evidence.
I think what you are asserting is that his statements are inadmissible, and what I'm looking for is your rationale for that.
I'd suggest you check Williamson again, and review the meaning of "hearsay," and review the Rules of Evidence to facilitate your explanation of why Zimmerman's testimony would be inadmissible.
If you aren't asserting that his post-incident testimony would or could be held inadmissible, then I'm curious what you see as the ramifications of categorizing any Zimmerman statement, either during or post-incident, as self-serving.