Apparently it's not _that_ easy.
300W/hour is more like 1000kcal/hr. But even so, 300W for 20 minutes of a 1hr ride is not HIIT. Depending on your FTP and how big/small you are, that might be an easy threshold workout, or a pretty tough VO2Max workout. But when they talk about HIIT, they mean all-out efforts, think 15-20 second sprints.
The real problem with HIIT is that most people don't do it right, the intensity required to due true HIIT (eg Tabata protocol) is very difficult. Most people are unwilling or unable to do repeated maximal efforts like that.
Your math is off again. My last 4hr ride was over 3000 kj; granted that was at about 210W, but even so your numbers for the 120-150W range are way low.
It's off by 3.6 as I acknowledge above