A "flat no" is a no, and it is not even up for discussion.
Think of how we sound in English when we say "NO" how flat that sounds when we really mean it. Not a raised "nooOOOoo" ♪ that sort of has a sing-songy sound to make it sound like it's not-really-a-no. It's a serious NO. Flat, no cute up-tone. Just NO. A flat NO. :)