The purpose of the trial was to avoid diseases caused by faulty DNA transmitted via the mitochondria. Mitochondrial DNA is only 13 genes out of 20,000, and is transmitted separately, but in some cases can cause disease. The third person here was a woman who donated her healthy mitochondria & its DNA to a nucleus where the existing male/female nucleus was damaged.
Will swapping out some of the 20,000 core nucleus genes be a future development? Perhaps, but maybe it will make more sense to have them gene edited, and not get transplants from extra people.