I can only see this being effective if the Nginx instance isn’t also responsible for reverse-proxying the frontend traffic, and if it’s not running on the same server as the frontend or backend (ie. decoupled from the infrastructure serving the stack). Haven’t looked at the article yet but I’m assuming they would recommend provisioning the infrastructure with that decoupling, or it wouldn’t make much sense.
You shouldn’t be charged for unauthorized requests to your buckets. Currently if you know any person’s bucket name, which is easily discoverable if you know what you’re doing, that means you can maliciously rack up their bill just to hurt them financially by spamming it with anonymous requests.