I think that's essentially my point. I don't think the question becomes 'What did Tracy say?' until Tracy denies saying it. Am I wrong?
ETA: I should say, until he denies saying it, or says something else that is impeached by his saying it.
No. The character of the question stands from the start. It's not hearsay unless offered as proof for the contents. But you are on to something, I think. The question "What did Tracy say" isn't relevant in a vacuum.
It's not impeachment that makes Singleton's and Serino's answers relevant. That's easy to see, imagine if Tracy had agreed with their testimony. Then all three would corroborate what Tracy said (which is different from an inquiry as to the truth of what Tracy said).
I think what was going on was a bit of horsetrading. It's difficult for most people to understand the hearsay distinction I'm describing, and, in context, it is highly improbable that a jury would not take Serino and Singleton's observations as Tracy offering "not my son" for the truth of "not my son."
So, the state argues that offering it for "what he said" and offering it for "the truth of what he said" can't be distinguished in this case, so it's inadmissible. Or some other smokescreen argument, doesn't really matter what the argument is.
What the state got was some contradictory evidence, so at least the jury is faced with a conflict over "what he said", which also creates evidentiary conflict over "the truth of what he said." Who you gonna believe, the police or Tracy? Who you gonna believe, Tracy, Sybrina and Jahavaris; or Zimmerman's friends and mother? IOW, instead of being faced with evidence that Tracy said "not my son," the state got Tracy on the stand to say the cops misinterpreted his remark. Basically, there are competing and contradictory pieces of direct evidence on exactly the same fact - "What did Tracy say?"
In the grand scheme of things (outside expressing truth in a courtroom), it's probably a good thing to give Tracy an opportunity to clearly join Sybrina in the scream ID issue. I don't think having him on one side or the other makes a difference in the outcome of the case, all the action visible to the jury makes his testimony equivocal, even though he gets to express conviction that the police got him wrong, and as far as he is concerned, that's Trayvon screaming.