The problem described here is that developer X creates an API key intended for Maps or something, developer Y turns on Gemini, and now X's key can access Gemini without either X or Y realizing that this is the case.
The solution is to not reuse GCP projects for multiple purposes, especially in prod.
You are also wrong in saying there are no projects that could reasonably have a safe api key made unsafe by this exploit.
One example, a service that has firebase auth must publish the key (Google's docs recommend). Later, you add gen ai to that service, managing access using IAM/service accounts (the proper way). You've now elevated the Firebase Auth Key to be a Gemini key. Really undeniably poor from Google.
[Edit: It's likely that you intended to reply to this comment: https://news.ycombinator.com/item?id=47163147 ]
It shouldn't be enabled by default on either one.
Of course, Google is full of smart anti-fraud experts, they just handle 80% of this shit on the back-end, so they don't care about the front-end pain.