Lifesum in-app purchases are, unfortunately, not eligible for Family Sharing at this time.
The information shown on the App Store listing is referring to the app itself, a default App Store feature, and indicates that family members may also download the app from the App Store.
In order to use Lifesum and Premium, each single user needs to create their own account and purchase a separate subscription for it.
This can change in the future but at the moment we do not have any technical solution for sharing a subscription.