Someone with root can run ostree admin unlock --hotfix to make /usr writable. Someone with root can also delete all restore points.
It would be strange for them to call it that if it actually means “completely irrelevant from a security perspective”.
See the comment by superkret.
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Which is what led to the further discussion of root making that not so.
I don't believe that to be the intent of the OP's comment, given their second sentence, but they are welcome to state otherwise. I just don't want them thinking that an immutable distribution gives them some kind of bulletproof security that it doesn't.