I think the myth's origin is that if you start fasting/ really cut calories you'll notice a huge, immediate drop for a day or two - a lot of that being that you're losing water weight. Like going from a night of eating/ drinking, and then fasting the next day, I can see a 5 pound difference.

Then that stops, and suddenly your scale isn't showing you the same drastic win as the first few days. You might reasonably think "I guess my body has adjusted".

That isn't to say that your body doesn't change how it functions in order to save energy, obviously a lot changes when you've exhausted your glycogen stores. But I think a lot of the "drastic" results for people hitting a wall after a few days of fasting is based on that initial huge win.