I think the daily changes in how "in shape" you are reflect how much water you have in your body from eating carbohydrate. Nobody loses a noticeable amount of fat in one day.

You lost weight by decreasing the amount of energy you were eating. Low-carb diets are an effective way to do this, because they are generally restrictive (less variety means less overeating) and because protein is very satiating. But there are other ways, and yes, some of them involve eating a lot of carbohydrates. The best thing is to choose something you can stick with. In that sense, your advice is not good, since 50-60% protein would make a lot of people want to barf.

I would go so far as to say there is no such thing as a diet that is "healthy" absent particular health goals. If you are overweight, one way to become healthier is to lose weight. In that sense, a low-energy diet is effective, independent of what foods it contains.