Okay...this sentence is probably innocent, but after reading the "Girls don't like to gain weight" sentence and thinking about the implications for eating disorders (read my comment on that if you don't know what I mean) it's hard for me to see this as anything but someone who doesn't understand anorexia and other restrictive eating disorders telling someone that has one "Shut up you don't have a problem; you're just losing weight and that's not an illness, you special snowflake." And of course this person wouldn't understand that it's not the act of losing weight that's the problem (though it is a lot of weight and is causing other health problems which is bad) but the mentality that keeps people trying to lose weight even though it's unhealthy that's the problem. If that's the case, I just feel the need to say that's a mental illness and it's not the same as a diet. I'm just saying this just in case; I don't think the person writing it meant any harm but the thought is in my mind from the other sentence.
Also the English version should say "Losing weight is not an illness" as a translation (adding "an" between "not" and "illness").
I think this sentence has more to do with the growing problem of obesity in East Africa. Traditionally, to be fatter meant you were wealthy and healthy, but now people are getting beyond a good size to obese as countries develop. Weight loss was most commonly seen when someone was ill, and it is probably still culturally associated that way. For example, in Tonga, it is a compliment to tell someone they are growing fat, but there is no word for being skinny. The closest is "emaciated." You can see the real difference there.
The literal translation of this is 'weight loss is not a disease'. As a doctor, if I see this from a medical point of view, this statement is correct because weight loss itself is not a disease but is generally secondary to certain diseases. As such, when you state a patient's diagnosis, weight loss would not be the diagnosis itself. It is a consequence of the disease whether that be an eating disorder, cancer, etc. I can't be sure if the creators of the course meant it in this way, but if they did, then it's not wrong and is being misunderstood.