He probably wanted to say "I'm and wasn't kidding", and wanted to keep it short and brief. I see no serious ground for rejecting this (correct) answer. There is another possible interpretation, but I prefer not to say it, and leave it up to you to think a bit about it.But I give his answer low credibility because he said "...I'm wasn't kidding." Sorry, you'll have to throw it out using your own credibility rating.
