My memory of Google US employment legalese - note I have not worked for them for over 7 years and am not speaking for them here - is that they acknowledge limits to their IP assignment provisions which are consistent with California law. Any Googler who is confident that those limits protect their ownership by default of their side project does not have to seek Google's approval in order to own it, even according to the contract wording.

But Google's business is so broad that it's often legitimately debatable (and sometimes beyond the knowledge of the Googler doing the work) as to whether something would be in scope. So getting their approval, which can come either with explicit assignment of rights back to the Googler or explicit permission to release under Google copyright, is often the prudent approach to minimize undesired risks.