Have to be fair RN, the rating says that below 5°C , and I quote, "In the risk range (-8° to +5°) a strong sensation of cold has to be expected. There is a risk of health damage by hypothermia".
It does not tell you to be sensible and wear warm clothes as well or cover the sleeping bag with another layer. Nor does it tell you to sleep on a raised matress/bed and not directly on the floor or ground. I'm sure the more serious propositions for trekking have been 'tested' with some scientific aim in mind but can't really tell you what you will experience, you might like to sleep naked for example.
I think it's more a disclaimer, like a paper cup containing coffee might carry the warning "contents may be hot!". Told you so!!!