He's not exactly Google PR, he's a high-ranking Linux maintainer. But, as OP responded in a sibling comment, it's in Google's interest to make him able to keep

He's not exactly Google PR, he's a high-ranking Linux maintainer. But, as OP responded in a sibling comment, it's in Google's interest to make him able to keep doing that work - that's essentially why they hired him. He's okay with Google owning the copyright to the OSS he works on, as long as it gets contributed, and that's aligned with Google's interests. He's also been at Google for almost a decade, and if Google is in any way competent, he has no incentive to be looking for other jobs (and reading their employment agreements). I agree with you that in fact most other tech companies make it pretty easy to own the copyright on work you do on your own time.