Many apartment buildings will open certain sections for rent that are finished while they complete the construction and furnishing of other sections.  This is especially true of larger apartment buildings.  So "fully open" means that all of the units were completed and ready for tenants to move in, i.e., the construction/renovation was 100% complete rather than still in progress.  "Fully occupied" would mean that all the units were actually rented, which has not happened yet.