I mean, if it was done on non-Google hardware, off Google property, and using resources that you paid for / own, then I can't see any problems with that. They can still argue that it competes with their core business, but they would have to provide some justification if they wanted to fire you for cause or have the IP assigned to them. I guess they can just terminate you for no reason (I think?), but that's the risk you take.

The possible need to prove that an employee's work was done on company time (which is a vaguely-defined notion for US salary employees in the era of remote work) or with company hardware is a huge hassle for the employer, which can probably shed light on why they'd use blanket assume-ownership-of-copyright agreements, so long as employees are willing to sign such agreements.