No. He didn't cite a "brief," he cited a decision by the United States Supreme Court on a relevant legal issue. That's an enormously important distinction. Briefs are irrelevant and, absent a few unusual circumstances, you'd almost never cite a brief from one case in a brief in another case. Decisions by the Supreme Court, on the other hand, are the single most authoritative source of common law in the United States. Often they're directly binding, even on state courts in federal constitutional or statutory matters, and even when they aren't binding they're enormously influential. And a given decision by the Supreme Court-- HLP being no exception-- deals with discrete legal issues that bear only a tangential relationship to the parties in the case. When the Court's exposition of a legal principle is analogous to an issue raised in a subsequent case, it's both appropriate and common practice to cite the Court's earlier decision. No one cares who the parties in that earlier decision were, and certainly no one interprets a citation to an earlier case to be drawing a moral equivalence between the parties in that case and the current case. That just isn't the way it works.