That he said it into a mic isn't a problem because it's discoverable, it's shocking because it suggests it wasn't an offhand comment, it was something he thought about before saying it out loud.

Evidence entered into the record in court cases is public by default unless the judge takes action to seal that evidence. Unsealed evidence is used to generate news, compiled for research, reviewed for commentary, etc.

Usually in these sorts of cases (where all sorts of sensitive info will be put into the record) there are agreements made between all parties before the trial about what will be shared publicly and what will be kept confidential.

So, he's not wrong. Usually there is an agreement in place governing what information will be shared, how it will be shared, when it will be shared, etc. But there wasn't an agreement in this case, and it's weird that he's acting like someone should have told him instead of acting embarrassed that he didn't know that.