Do authors normally check their own papers for plagiarism?
I've never put such an assertion in one of my own cover letters, and would find it very strange to do so.


It's assumed that I should be behaving honestly and avoiding both plagiarism and self-plagiarism.  Saying: "really, I'm not plagiarizing!" just makes me wonder why the person felt they needed to say anything.
If I'm not behaving honestly, why wouldn't I lie about the results of plagiarism-checking too?


If I saw this as a reviewer, I would think it reflected an odd but almost certainly irrelevant thing about the authors, and then proceed to ignore it.