Although in general I do agree that the statements in the post are correct and backed by scientific results there's more to it than that. While HIIT is a very good form of exercise you are taking marathon runners and bodybuilders in this case (which are the extremes of each type) vs people looking to get healthier or gaining that beach body look.

Let's see the other side of the story
Note : I have a degree in Physical Education and Sports Science and am a Track and Field Throws Specialist

Muscle type fibers while % wise are usually uniform across the general population world class athletes with good/long-term training (10-12 years) vary a small % wise but big enough to make a difference in their relevant sports.

A marathon runner looks to increase VO2 Max for his specific sport. Using HIIT (year round) in the programming will not gain the needed results to be competitive in the sport. The heart also evolves differently to be able to pump more blood (and oxygen) with less pulses to the muscles (thinner walls).

An olympic weight-lifter or powerlifter looks to increase fast twitch muscles to a degree which HIIT might even hinder their performance unless they need to drop a weight-class fast or are in the start of a programming cycle. The heart in these athletes also evolves differently to be able to pump blood faster to the muscles (thick walls).