Interesting, but as there are other charges involved the negative interest rate is a marketing ploy. The examples given on Jyske bank's website involve repaying more than disbursed https://www.jyskebank.dk/bolig/nyheder/realkredit-med-negati...

There was an HN discussion about this where several Danes mentioned that these loans still come with assorted fees and taxes which effectively amount to a low, but still positive, interest rate.