While you are correct, any system is compromised if you have root, so isn’t that irrelevant at that point?
The original context for the comment chain was:
Because even if an attacker could gain access even as root he cannot modify system files.
So no, it's completely relevant.
Your comment also contained
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.