Dt Support responded to this question in a ticket I have open on another issue, so I'll just repost that info in the hope it is clear to others that may wonder.
Bottom line is K2 is not supported in the latest version (2.8.5). There is a feature that you can link the registration listing to an article, however it has to be a Joomla article. If you're using K2 and use the K2 Items (articles) for your website content instead of the Joomla articles, then you are out of luck.
Unfortunately this is the case yes, I have wished for K2 Items integration like with Joomla articles for years, but it has not been a priority thing it seems.