If it were phrased like this then you would be right. The docs would give a false sense of security, would be misleading. So I went to check, but I didn't find such assertion in the linked docs (please let me know if I missed it) [0]
So I agree with the commenter above (and Github) that "editing the github action to add steps to download a script and running" is not a fundamental flaw of this system designed to do exactly that, to run commands as instructed by the user.
Overall we should always ask ourselves: what's the threat model here? If anyone can edit the Github Action, then we can make it do a lot of things, and this "Github Action Policy" filter toggle is the last of our worry. The only way to make the CI/CD pipeline secure (especially since the CD part usually have access to the outside world) is to prevent people from editing and running anything they want in it. It means preventing the access of users to the repository itself in the case off Github Actions.
[0] https://blog.yossarian.net/2025/06/11/github-actions-policie...