Not exactly. It's kind of like when the jury hears something, and then they're told to "disregard" it. Obviously they can't actually disregard it, but the form requires the instruction from the judge, and the jury can't later say they went and regarded it anyway, or it would be a mistrial - even though everyone knows they regarded it. Same difference here. The judge knows they're guilty, they even tried to plead guilty, as you pointed out. But if the judge goes and says he wants to put them away before they are actually convicted, he's demonstrating bias. And in a sense, a mistrial is actually justified - the judge is not only biased, but he's so biased he can't keep quiet about it, which is definite evidence of lack of control, if not intent.
Fair enough — cheers for that straightforward explanation.