According to Jat:

"Don’t worry! Primitive+ will still be releasing on Xbox One - it has not been cancelled, let’s dispel those rumours right now. It is fact coming fairly soon! Though, it is still a couple weeks away. The reason for the delay, outside of regular procedure matters (certification etc), is that we wanted to take a more careful approach with releasing this DLC due to the some of the bumps that we had run into when we initially released our first DLC, The Center onto the Xbox platform. A few of our recent patches have had significant changes which have led to some painful situations which we’re still recovering from in terms of issues that crop up post release. I know it sucks as you guys have been very passionate about playing Primitive+, we’ve seen the support all over social media and we’re truly grateful for it, which is even more reason why we want to do our best to ensure that when the DLC is made available, it does not suffer from those same bumps. We’re going to ensure this by putting it through it’s paces with more vigorous QA, and monitoring the official release on PC for any undetected issues, as well as conduct extra iteration when necessary. We are sorry about the delay, and it won’t be too long, and when it does become available, you’ll be glad we avoided the risk and hopefully we’ll have a smooth, bump, and bruise-free release"