It's "implied" throughout the whole post (or more like assumed that the reader understands this, because it's the basic premise of the problem). It's why they link to a post that explains the basic concept after a remark that "This describes our issue in more detail".

> tl;dr Google spent over a decade telling developers that Google API keys (like those used in Maps, Firebase, etc.) are not secrets. But that's no longer true: Gemini accepts the same keys to access your private data. We scanned millions of websites and found nearly 3,000 Google API keys, originally deployed for public services like Google Maps, that now also authenticate to Gemini even though they were never intended for it. With a valid key, an attacker can access uploaded files, cached data, and charge LLM-usage to your account. Even Google themselves had old public API keys, which they thought were non-sensitive, that we could use to access Google’s internal Gemini.

From Google themselves, in the Firebase docs:

> API keys for Firebase services are not secret. Firebase uses API keys only to identify your app's Firebase project to Firebase services, and not to control access to database or Cloud Storage data, which is done using Firebase Security Rules. For this reason, you do not need to treat API keys for Firebase services as secrets, and you can safely embed them in client code.

<https://firebase.google.com/support/guides/security-checklis...>

... or at least that's what it used to say, until they quietly updated the docs to say this:

> API keys for Firebase services are not secret. API keys for Firebase services only identify your Firebase project and app to those services. Authorization is handled through Google Cloud IAM permissions, Firebase Security Rules, and Firebase App Check.

> All Firebase-provisioned API keys are automatically restricted to Firebase-related APIs. If your app's setup follows the guidelines in this page, then API keys restricted to Firebase services do not need to be treated as secrets, and it's safe to include them in your code or configuration files.

Followed later by (in different section):

> Use your Firebase-provisioned API keys only for Firebase-related APIs. If your app uses any other APIs (for example, the Places API for Maps or the Gemini Developer API), use a separate API key and restrict it to the applicable API.