That analysis is addressing an argument that the Supreme Court could not overturn the finding that it was not fair use. That argument was based on the Seventh Amendment's reexamination clause. The Supreme Court held that the reexamination clause did not prevent it from addressing this issue, because the question is a mixed question of law and fact, i.e., it's a legal conclusion based on evidentiary facts. The Supreme Court cannot re-examine the "underlying facts," but the Seventh Amendment does not preclude it from deciding, as a matter of law, whether those underlying facts constitute fair use.