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 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, the in-app products are not eligible. Please let me know if you have any questions about this.
If you are interested in sharing Calm with your family, please reach out to our support team here to discuss other pricing options.