To the extent that they don't have that kind of credibility (I think I agree with your assessment) whatever the eventual result of Hinton joining the team should say something very strong about whether or not they will. If he finds that he isn't getting what he wants out of it, I think we can expect him to leave. If he ends up staying it is evidence that Google can hold the interest of the sort of people you would expect to find at places like Bell Labs.