Umbilical hernias are usually fixed through a small incision near the navel, and these are usually done by general surgeons. They seldom repair diastasis because they are not symptomatic, the repairs are considered cosmetic, and they require larger incisions.
Plastic surgeons often repair diastasis rectus as part of a tummy tuck. We already have a fairly large incision and have ready access to do this repair, and it adds to the cosmetic improvement for the patient.
There is no reason to suspect that there would be an increased rate of recurrence of an umbilical hernia repair related to the presence of separation of the rectus muscles (disatasis).