I guess that's what happens when you tell easily disprovable lies.
Establishment FBI directors pretending to be Republicans is nothing new either (James Comey was a Democrat 100%. He's endorsed Democrats but never Republicans. Same story with McCabe, whose wife is a Democrat). Well, really everyone who comes out of the FBI is a liar. Seems obvious they have nominally Republican figureheads like Robert "I admit I've never made a 'non-exoneration exoneration' before" Mueller to create the illusion of non-partisanship as the agency sides with Democrats on every single decision they make.
It doesn't matter if he's under oath because he would never be held accountable for lying.