HIIT tends to burn more calories, so by proxy more calories from fat, in a shorter time than steady state cardio.*

*Based on available research on energy systems and some somewhat crude math.

Even though it's a max of about 3-5 min of working time, the high intensity should burn more calories overall when compared to 30-60 min steady state. Since it's short and high intensity, you're using mostly the ATP-PC then Glycotic systems, so it's something like 70% ATP-PC, 20% Glycotic, 10% fat for the first 1-2/3ish minutes depending on the person. Then 60% Glycotic, 30% fat, 10% ATP-PC for the rest. Even though it's a smaller percentage, it ends up being a higher output of calories expended from fat.

There's also scant evidence about a thing called oxygen depletion that I'm not fully sold on and the research on it is iffy. But the idea is you build up such an "oxygen debt" within hit that your body overcompensates for 24-ish hours after and it raises your metabolic rate by like 5-10%. Again, far from proven but may exist.

A big side issue is that most HIIT programs aren't actually HIIT. They're some sort of mid-range cardio. You have to be able to keep your heart rate at like 90-95% of max for the on period and no lower than 80% in the off. Most online programs I've seen don't really care about that, but that's where research on HIIT has come from. Most of this research was also done on people in half-decent shape so for people going from nothing to HIIT theyre probably going to have a bad time.