The extension was specifically designed to notify Google employees about problematic reliance on non-Google services. Seems like her warning fits that use case. How is "warning about Dropbox" any different from "warning about X management consulting firm"?

Edit: Her warning literally said: "Do not violate NLRB notice. Googlers have the right to participate in protected concerted activities." (see screenshot in article, bottom right.) This is something that Google is already obligated to acknowledge under labor law. They're simply shooting the messenger.