Compare e.g. https://thebusinessprofessor.com/en_US/property-law/californ...
I bet Google lacks the copyright for many of the projects they claim.
[1] you fill a form with the general idea description, you explain succinctly why it doesn't compete to any Google product, and you acknowledge you won't use Google time and resources to develop it. Then a committee reviews it and in ~1 week you get your approval or request for more details.
It might work for people who chisel away at their own long-term one-person pet projects, but breaks down if you're used to quickly hacking away collaboratively in various groups. Especially if you're an active member of a hackerspace.
IMHO, such "forced labor for free" is a form of slavery. Slavery is forbidden by law.
If a developer want to publish his project under Google brand, then exchange may be considered as fair (project code for Google branding).
Good luck taking Google to court.
I went through the IARC process 2 years ago for a startup I was about to found in Tokyo, and obtaining clearance was easy, as you say. It's an extra precaution to avoid future litigation, but not legally required if you keep your personal coding activity clearly and provably separate from your work activity.
Source: I'm a Googler that went through the process before.
Edit #1 for all Googlers reading this: search "IARC" internally and you should land on the proper page to kickstart the process.