It's was her personal judgment that it should be posted in a chrome add-on rather than only by poster like is typical in most workplaces. But assuming her job was to maintain an add-on that informs employees of company policy, and assuming her job typically lets her act with personal judgment, yeah I think that's perfectly reasonable
In fact, you can find further support of her action from the NLRB FAQ:
https://www.nlrb.gov/resources/faq/elections/are-there-any-c...
"The employer is also required to distribute the notice electronically to unit employees if it customarily communicates with employees in the unit electronically, either by email, by posting on an employer intranet site, or both"
Again, it's her judgment to add the notification (which links to the Google intranet posting of the NLRB poster), but considering that Google customarily informs employees of policy via notifications in this add-on, and she is in charge of maintaining said add-on, it doesn't seem like an unreasonable judgment to add this notification