I don't know where you're getting this stuff. Please cite your sources if you are going to make declarative statements about medical outcomes. At least for a-fib, the only studies I can see discussing a-fib in relation to HIIT indicate that outcomes (and overall cardiovascular health) are better with HIIT than with constant pace exercise.

Also, even if one doesn't subscribe to the high intensity aspect of HIIT, pretty much every fitness guide indicates that improved health and fitness comes from varying your effort through various heart rate zones. I don't think there's really any debate about this.