For these con-men, words have no meaning, they are just sounds you make to get other people to do what you want them to do. Don't listen to his words, watch his reaction. First time I've ever seen a stress-indicator in Fauci. Watch his left hand stretching rubber bands like crazy; watch how rigid his entire upper body becomes and how locked his arms become when extended (all muscles tensed). This indicates extremely strong emotions being restrained. One interpretation is that Fauci is reacting like any normal person would when falsely being accused of lying -- by (naturally) becoming agitated and upset (and also trying to modulate the reaction to prevent an outburst). But Fauci does this stuff all day long and has done it for many years. Presenting the calmly apathetic face of a permanent bureaucrat, even in the face of political attacks and accusations, is something he has years and years of hands-on practice doing, not to mention full national attention for the past 18+ months. How amazing, then, that one lone Senator can get his hands shaking, lower lip trembling and his entire upper body as rigid as a stone.
Also, his language does have a couple giveaways. First, note that he rebukes Senator Paul directly, "if anybody's a liar here, it's you, Senator." Well, that's not how the world of Establishment power works, according to their own unwritten rules. You may hate each other as much as you want, but you may never ever disrespect the office they hold. A Senator is higher-ranking than any Federal employee, even Fauci, and for Fauci to rebuke a Senator in a Senate hearing, as a witness, is a direct contradiction of Establishment protocol. Of course, he won't be held accountable for it in the public view. But it may be a sign that things are breaking apart beneath the surface. Bold, yes, but it appears to me a step too bold.
"You don't know what you're talking about, Senator." Well, this particular Senator is also a doctor of medicine. So, yeah. He actually does know what he's talking about. You can play the "I'm the Expert(TM)(R)(c)2021 all rights reserved"-game with the masses; but the rules of the game don't work the same way between experts (even in different fields). You can say "I'm the expert and you don't know what you're talking about" but this just looks childish and defensive when you're talking to someone who is also an expert, in your field. Fauci seems to think that the rules of academic silo-ing generalize to every context. They don't, especially not in the political context. We've put up with nearly two years of overpaid disk-jockeys ... aka "cable news anchors" informing us -- on their own authority! -- that we're "not taking this seriously enough", and many other BS opinions painted as facts. Blatant propaganda. As if that injury was not enough, now we have Doctor Death Fauci telling another doctor that he "doesn't know what [he's] talking about", as though Dr. Paul just stumbled out of a lifetime of coal-mining into his Senate seat.
When actual academics disagree, as a rule, they couch their language in terms of mutual acknowledgement of the qualifications of the other -- "I know you've done your research, and I understand how you reach your conclusions, but this is the specific point at which I think you are making a mistake in your reasoning, and here's why..." That's what healthy academic discussion looks like when it is not poisoned by politics. Fauci tries to keep up the facade of being an academic (despite merely being a career bureaucrat), and it's very telling that Senator Paul was able to crack that facade...