High or low intensity specific periodization may or may not work for different individuals. For some this may seem obvious, for others controversial. The reality is that we are learning more and more about individually driven training periodization, and that is where the future is going.
HIIT benefits could be somewhat overstated, unless they are controlled for total load. PLUS, if you consider some “extra”-training for a couple of weeks (e.g. post study intervention), we see that benefits often equalize between experimental groups. So HIIT is good, but likely not as “FANTASTIC” as everyone makes it out to be.
When you come across an athlete that is underperforming, and even athletes at the highest level in their sport, consider ALL the potential stressors. Fasted training, low carbohydrate availability training, Norwegian method threshold training and HIIT all have their place. But too much of any of these stressor leads to maladaptation and underperformance. As so often is the case, a balanced method of training done consistently over time is what returns an athlete to high levels of performance