1. It's not that lean people can't lose weight. It's just that as one gets closer to their limit on minimum body fat, it becomes more difficult and it's more likely to take unhealthy means to accomplish it--like caloric deficits that work for short-term weight loss. This article might help you understand the mechanics: https://medium.com/illumination/why-cant-i-lose-those-last-5-pounds-9b37272ad43f
2. The point of fasting for most overweight people is to lose weight. Lean people tend not to do things to try to lose weight... When lean people fast it's generally for other reasons.
3. And how was satisfactory compliance measured in this study? Self-report? The link you provided didn't discuss any compliance measurement or control.
4. Don't expect me to believe you with regards to what's behind a pay-wall... If you have access, tell us what controls were in place.
5. But alternate day fasting protocols don't recommend overeating on fed days. So the Alternate-Day fasting protocol was not tested in the study...
6. Insulin has everything to do with inhibiting fat oxidation. Are you not aware of the role of glucagon in fat oxidation and how it is inhibited if serum insulin levels are too high? That's why insulin resistance is such a problem when it comes to weight management. This article deals with the topic: https://medium.com/illumination/whats-weight-got-to-do-got-to-do-with-it-ffd50d587db1
In your scenario, when extra insulin is secreted (there is always a base level of insulin in the blood) to handle an influx of glucose in the blood, fat oxidation will stop until serum insulin levels return to that base level. How quickly one's insulin level returns to this base level will depend on how much insulin needed to be secreted to manage the glucose put into the bloodstream. An insulin resistant person will require more insulin than a non-insulin resistant person to rid the blood of the same amount of glucose--thus, keeping their serum insulin level high enough to inhibit the use of body fat longer than the lean person. It can take so long for some people that they don't have access to body fat between meals--these are the people who need a snack between meals every day or they get "hangry," or a headache.
If you consistently live with a caloric deficit, after sever weeks your body will defend that deficit and you'll end up gaining weight... The article linked in #6 discusses this as well.
Yes--insulin just does its job. But part of its job in managing blood glucose is to store it as fat in fat cells (the transformation of glucose into fat is done by the liver--but insulin does the transport). And while it's doing it job of managing a flood of blood glucose, it inhibits glucagon from doing its job of regulating fat oxidation.