I may be wrong here, but the speech doesn't sound right on this one when you listen to the audio. It shouldn't be pronounced as "knee" but more like "nay".
(Disclaimer: Whilst I'm obviously not fluent in Dutch, as I'm taking this course, I've lived in the Netherlands for over 3 years and am married to a Dutch woman. I've never heard anyone pronounce it "knee".)
lingoingo stated a fact that was true at the time, that "nee" was the only suggested translation of "nee". This is a fact, so you can't really agree or disagree with it, it's just true.
It only makes sense to say you agree with someone when they are stating an opinion. So, if someone said "This sentence is really weird." it would make perfect sense to say "I totally agree with you."
But in this case, it just doesn't fit.