I can see some potential flaws with this. For one thing, how do we (legally) define a startup? Another issue is that existing companies (who wanted to hire these people outright) might do an "end run" around immigration laws by buying these companies, thus acquiring those workers. It still would not necessarily prevent US workers from being displaced.
These points aside, I think PG has a rather optimistic outlook on the potential success of these companies, particularly with regards to creating jobs. Even in the best of economic times, the vast majority of startups do not grow anywhere close to Google's size.