I think what they're getting at is this: While you may have lose 20 lbs, only some of that is fat. There's other things to account for like water weight, muscle loss, even food you've just eaten. (Obviously, if you've just eaten a heavy meal, that's going to weigh you down and vice versa.) You might weigh yourself next week and find out you've gained some weight back. Basically, when it comes to weight loss, your day to day weight doesn't really matter as long as you continue losing weight over time.
Also, I forgot if this is true or not, but I think the rules they give don't apply if you're obese.
Also also: "Losing weight too fast is a myth."
This statement is kinda vague. Like, if you basically starve for a month you're going to lose weight, but according to conventional knowledge, it'll probably be unhealthy. Though I think I read a news article about a study obese patients who ate nothing and only took vitamin supplements for a month or something and had no adverse side effects, so I guess YMMV.