I was initially tempted to side with Google, a code review process is not just rigorous enough for catching features being sneaked in.

However, it looks like the sole purpose of the extension is to warn employees about security issues in websites, and this engineer coopted that to show a union forming message.

The final paragraph suggests that there's legal precedence for doing this:

“You’re allowed to put posters up on the wall to help organize your workers, and this can be seen as kind of a digital extension of that,” said Veena Dubal, a professor at the University of California’s Hastings College of the Law in San Francisco. “Whether or not you’re in a union or there’s a union being formed, the law protects organizing activities by employees in the workplace organizing to improve their working conditions.”