We obviously don't know the details yet, but this case does sound like it's on the more frivolous end of such charges. If they actually wanted to prosecute on it, they'd need to convince another judge/jury that this judge didn't just make a mistake about where the targeted person was supposed to be right then. This kind of prosecution normally involves comparatively more concrete things -- say, someone claiming to have no idea about a transaction and then the feds pulling out their signature on a receipt.
Of course, this could be a case where the judge knew the person was in a waiting room because they'd just talked to them there on camera, and then deliberately told the ICE agents they were on the other side of the courthouse while they were recording everything.