More on this Mystery Witness: Levin thinks the witness is Andrew Miller and Mr. Miller challenged the constitutionality of the appointment of Mueller with 'Appointments Clause' of Constitution. Probably challenging the subpoenas by challenging the constitutionality of Mueller. Grand Jury Cases pose a threat to Mueller because they can serve as vehicles to get questions of his authority and legal legitimacy before appellate judges relatively quickly. An appeal to DC Circuit Court is now in progress. Could this challenge go all the way to the Supreme Court?
Levin appears to be reading from this article in the first half of his video.
He, then, makes the above comments (paraphrased),
and others, on the matter.