Really? Google promised these people that if someone else used their correct username and their correct password, that other person wouldn't be able to log into the Nest camera?

I didn't see anywhere in the article that says Google promised that, and from what I know of security that's impossible. The article did say Google offered a way of increased security, an answer the consumer neglected to use and says isn't enough. (edit - says with absolutely no knowledge on the subject that 2FA isn't enough)

In your opinion, what should Google have done differently here? And where did you see that Google promised no one would be able to log in if that person had the correct username and password and 2FA wasn't enabled?

Huh? Where do you see anything about Nest telling the consumer this wouldn't happen? They even encourage 2-factor in the setup process of the camera. I've never seen Nest say anywhere that the camera is 100% secure.