Why does Judge Judy testify?

[rassomaxa]

On innumerable occasions, a plaintiff or defendant will testify "He
knows .." and Judge Judy will assert, on the contrary, "He doesn't
know."

According to my understanding, in a bench trial, the judge determines
the facts. How does a judge determine that one party doesn't know a
certain thing unless that party so testifies?

It would certainly be my position that my adversary knows a certain
thing FROM ME (because it is true and I told it to him) but chooses to
ignore that knowledge.