Some people seem oddly surprised at this revelation? I thought that there had already been plenty of hints towards the sci-fi for Tsukasa's immortality origins rather than magical or divine.
Also the fact that this was a tucked away extra rather than the focus of a full chapter, at least at this stage, is appropriate as it is still a rom com overall and not a sci-fi mystery in focus. I feel if it got explained too much, either now or even later, it would lessen the feel of the background mystique to the detriment of the story as a whole.