Earlier quoted context omitted.
Yes you are missing something important. Once they've tied the browser ID to your personal account they can track you across all google properties, even the ones that you didn't log into.
I still don't understand. When I log into gmail, it logs me into all Google services. If I am worried about being tracked, surely my first mistake is logging in in the first place? Or visiting in the first place? After all, even if I click "log out," I'm only trusting Google that they unlinked the browser state from the account. If I trust them to do that, I don't see why I shouldn't trust them to ignore this experim…
Good luck completing a google captcha without a Google account or using Chrome.