So if a husband says monkeys flew out of his wife's butt, it's a fact because he was a witness to it.
It's not a "fact" in the sense that's its truth is beyond question.
It's admissible, if it's relevant, and not barred by the rules of evidence.
It's not due any particular weight by itself, i.e., the "fact" can be contradicted by other facts in evidence, and the witness's truthfulness, his ability to observe, or to recall, all are subject to impeachment on cross-examination.
The reason Dershowitz is claiming this isn't hearsay if because Michael isn't testifying as to the truthfullness of Terri's supposed statment. He's only saying it happened. For example, if you hear someone say, "Once I kill my wife, I'll inherit millions," its not hearsay, assuming you actually heard it. Michael claims Terri said these things about not wanting to live that way - he's not testifying that she actually meant it. The truthfullness of Terri's statements are not at issue.
At least, that's how Dershowitz is presenting it. I'm not familiar with Florida law.