While I agree with you that this is not a major effect, it is an effect nonetheless and it's due to changes in hormones. Dr Fung has a blog where he discusses it using an analogy https://idmprogram.com/how-to-control-the-body-weight-thermo....

I'm not the OP, but I'm also not convinced that the set point exists. Regardless, fasting is a better way of losing weight than caloric restriction IMO since it lowers insulin, but to each their own. Either way, the laws of thermodynamics haven't changed, it's just how your body responds, and we're all unique in that sense. Whatever works for people, so long as they're losing weight if they're obese or overweight.