Google does not rely on a user being logged in. Go to adsettings.google.com in a logged out state, for example. I'm not sure what you're referring to.
The ad industry has had these opt-outs for a while, but you have to set opt-out cookies on about 500 sites, so it's not practical. DNT solves that problem, but the industry won't voluntarily adopt any solution that has any realistic chance of making a difference.
I opted out.
I pressed the trash can button in Firefox Focus.
I reopened the site. It said I was opted in.
Their "disable tracking" button simply does not work.
I'm not enabling cookies so I can opt out and they can just buy my tracking information from some other ad network. There is a reason I disable persistent cookies and want DNT headers to be legally binding.