Trial Testimony and Evidence

This is incorrect. For example, a criminal in a court proceeding might give evidence against another criminal, involved in the same crime. It is common for the criminal giving such evidence to be taken seriously.

i'm not sure why anyone would assume this was the only piece of evidence presented in the trial of this person.

"because the people on trial said so" does not really constitute evidence.(not that they are in any real way on trial mind, they got to vet and prepare for every question before hand.)

No, in the same way lawyers and the judge can decide ahead of time not to admit an unqualified expert witness, or to dismiss the jury for someone like Darrell Brooks representing himself by bringing up irrelevant information and hearsay which is inadmissible in court. If someone says the wrong thing in front of the jury it is somewhat difficult to erase it from their brains or to convene a new jury.

Experts offer evidence and testimony, and the offender is given a chance to refute.

In court? By calling a credible witness, perhaps an expert, who says “X isn’t occurring.”

Unless you have no reason to believe the testimony will matter.

This has been tested in court as a defense. Try to guess what the result was.

There are witnesses and the jury evaluates them.

The quote( not your comment ) is a logical fallacy. Even if was not accepted as hard evidence in the court, it was spoken, and has influenced the jury.