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The filesystem itself is also read-only.
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.
Your comment was already from the position of if an attacker could gain root access. My responses were to that directly, and nothing else.