On a 5 hr bike ride your body is going to go into a similar state as intermittent fasting with all your glycogen being burned off. Similar to fasting this frees up energy and allows the garbage in your system to be burned off.
Still seems a little too "high level" to actually believe though. In any case, I still do IF by skipping breakfast, mostly because I'm lazy, and after fasting for so many years hunger doesn't really bother me that much.
A 24-hour fast is different; your body will burn fat at sufficient rate to avoid glycogen depletion (it's pretty simple; the harder you exercise the less energy comes from fat and more from 'carbs').
From a weight-less perspective though all that matters is calories out exceed calories in - it's quite possible though that fasts end up making you less hungry than hard exercise does.
Long-distance cyclists don't worry about any of this though - the approach is actually pretty simple:-
Press food to face.
Suck.
Repeat.
Edit: formatting.
The numbers suggest 60 to 70 per cent of max heartrate (which is pretty easy) is the sweet spot for general fitness and fat burning (i.e. weight loss). For improved endurance, spend a lot of time in that zone; for improved performance spend shorter periods (well) above it.
I'm not qualified to judge the literature as a whole, and it appears to be an open question whether this can be translated to actionable advice, but we're not in complete bullshit territory here.
What precisely is this?