Effective for what? Body composition? Fitness? If the latter, please explain why all elite endurance athletes do the overwhelming majority (>95%) of their training at low intensities. If the former, well, elite endurance athletes sure aren't fat.
Second, what elite athletes do often does not translate to what laypeople can and should do. Most elite athletes have a tremendous gene advantage over other people, not to mention the time, energy, diet, coaches, etc., that allows them to push beyond what the average person can tolerate. It's almost ridiculous to say, "Micheal Phelps does xyz in the pool and you should too if you want to improve your health in the most efficient way possible."
See https://bodyrecomposition.com/training/endurance-training-me... for an example. The German track cycling team does 90+% of their training at low intensities. For an event lasting 4 minutes. The simple fact is that at the highest level, all pure endurance athletes do mostly aerobic work.
The reason I separate elite athletes from others is that they are genetically different than the average person. To be an elite athlete in a super competitive sport, you are not representative of the normal population. You can likely train harder and longer and most likely have some type of other genetic advantage. It's far better to focus on studies that look at normal people rather than elite athletes.
Anyway, to go back to your original question, asking how HIIT is more effective, you are right that it is not necessarily effective at every endeavor. But literature shows that for the average person, doing 10 minutes of HIIT can be as effective in losing weight, gaining muscle, cardiovascular fitness, etc. as doing something like 30 minutes of low-intensity exercise.