Ah, the old "Let me track you using this cookie so that you don't have to sign in and let us track you" trick! I'm guessing between IPs, browser fingerprinting, and usage/viewing habits it's beyond trivial for google to know the computer opting out at 3:00pm today is the same one that opted out this morning before clearing their cookies, and the same one that opted out yesterday for 16 hours and the same one the day before that, etc
Nope, google is legally obligated to not do that, and in fact they don't. Google has much more to lose in this than you do, and a strong incentive not to lie.
How'd we ever know if they did? Are there independent audits or would discovery that they have been doing that depend on a whistleblower? If we know anything about corporations it's that they have zero problems with violating laws as long as they'll have mode more money than the slap on the wrist fine or class action settlement they'd pay out assuming they are ever caught in the first place.
I disagree that they would make more money by lying directly and explicitly like that, and yes it would be very difficult to cover up a lie like that. Literally millions of people would know, because any employee at Google or anyone who inspects the code of any google web app would learn of the lie.
As for me, I'll stick to youtube-dl