Googler here (though not a lawyer and I don't speak for the company). I agree that the OP's assessment that you quoted is incorrect in general, for the reasons you mentioned. The Google-internal process that OP is alluding to allows an employee to seek positive assurance from Google that the company will not attempt to claim ownership over the project. It's a way to get agreement up-front that both parties agree the project is unrelated to the business. You don't give up any rights by not going through the process, but you might discover later that Google believes the work is related to the business, and believes that it owns the IP.

However, there is another completely separate process that allows you to open-source your code while having Google retain copyright. It's much easier to get through (turnaround is just a few days) and lets you release the code under a permissive license like BSD or Apache2. I write a lot of open-source code on the side, and I've used this process to get approval for all of it. It makes no difference to me whether Google technically owns the copyright.

I would add that Google's open-source policies are a huge plus in my book. It's a night-and-day difference from Amazon where I used to work. Trying to get anything released as open-source at Amazon was like pulling teeth.