After it took place, Christ's sacrifice had never ceased to be a part of reality. But Christ's sacrifice did not continue to be always a part of the present. (Christ's sacrifice is present during the Mass, but there have been times, since Christ's crucifixion and resurrection, during which no Mass was being celebrated.) Hence, the present and reality are not coextensive.
Whether this contradicts presentism depends on what one makes of the imprecise predicates "is a part of reality" and "is a part of the present".