I live in California, and they can. The key there is that is isn't "Unrelated to your work" it is "Unrelated to the work of the company." Also phrased as "The company's business." Google literally claims they could be in any business[1] at any time so anything you work on belongs to them. Further, as my lawyer pointed out to me at the time, California is an 'at will' state so they can fire you for any non-protected reason, one of which is 'working on things and forcing them to sue to get rights to that work product.' It isn't a very balanced situation from a power dynamic. My advice to anyone at Google (as it was when I was there) do not work on anything that you might later want to develop further while working at Google. If you really want to work on this thing, quit, and then start working on it. Otherwise you are at risk of the "success disaster" where your side hustle is suddenly worth something and at the same time you're being told to hand it over to your employer or be fired and sued at the same time.

[1] They rationalized this when I was there with their "20% time rule" which was time to work on what ever you wanted, but working on whatever you wanted still belonged to Google because they had a unlimited right of first refusal to productize whatever it was you worked on.

I think you're overstating. If it's on your time and your equipment and unrelated to your work, but it is related to your employer's business, it's not obvious.

If you work for a big tech, there's a huge range of stuff that's related to your employer's business, which means you have to tread carefully.