IIRC they added it to one of the utility extensions that many Google employees install, although I no longer recall which one.
Kind of funny seeing maybe some google naivete and the "do no evil" thing come back to bite google.
When I was in high school I had a fair bit of fun going outside the permitted guidelines. By college I'd developed a reasonable sense of boundaries with other folks equipment / jobs with a trust element.
If I pushed a pop-up notice out company wide with a message my employer didn't approve of, I'd definitely be expecting to be going out the door rather than doing the litigation process.
Having worked a bit in govt interesting to see the NLRB pursue this, in govt my experience was a REDICULOUSLY dim view would be taken of something like this.
If it were relating to anything other than labor organization, it would be a no brainer. But there are special legal protections for labor organization, so I not sure it's clear cut in this particular circumstance.
There are a lot of employee-written extensions at Google, published in an internal Chrome web store. I even wrote one in a few days and published it. Some of them are owned by teams in one way or another, but a surprising amount of developer tooling at Google is somebody's side project.