Apple does not allow sharing for any in-app purchases or subscriptions. As such, Calm subscriptions are not eligible for Family Sharing. You can read more about Apple's policy here: https://support.apple.com/en-us/HT203046
Because the Calm app itself is added to the Family library of apps once it's downloaded by a family member, it is considered a 'Family Sharing' app, which is why the App Store details state that. However, in-app purchases, including subscriptions, are not shared.
If you are interested in sharing Calm with your family, please reach out to our support team here to discuss other pricing options.