It probably came from some out of scope call out in the original prompt, or a redirect. I have found that AI really loves patting itself on the back like this.
It really really loves mentioning what something "is not" right after it just said what it is. I don't like this pattern and see it more and more in READMEs, probably part of recent Claude RL. I don't think its as prevalent in the zeitgeist as the emdash, "Its not X its Y" and the emoji bullet list.