In the family guidebook...
I note that while the Family Guidebook is indeed referenced as an official resource in a few places (for example, in the Additional Resources section of the "My Calling as an Elders Quorum President" webpage), I think that not everything in this guidebook is necessarily current. For example, in the "family guidebook - priesthood ordinances and blessings - baptism" section, it states the witnesses must be two priests or men holding the Melchizedek Priesthood, which as we all know is not current policy.
I'm not saying the quote you referenced about the sacrament is incorrect, just my perspective that what's listed in this particular guidebook may not be as official as other materials the Church has available.