Planks create what in fitness what is know as "abdominal bracing." You need to have an adequate amount of strength in the Transverse Abdominis to do correctly.
After pregnancy, planks can be more difficult then they look and should not be done as a first rehab exercise for diastasis recti.
Many pp women with DR have little to no ability to contract the Transverse Abdominis, or TvA, our body's internal "girdle."
During exercises in the quadruped position, the TvA must also contend with the force of gravity. If this muscle lacks adequate functional strength, then the abdomen will drop downward, placing tensile forces on the already lax mid line connective tissue, exacerbating diastasis.
Planks are also an isometric exercise. Isometric exercises ONLY build strength in the body position which you to them in (muscle specificity theory).
So while planks certainly have an important place in core conditioning, they are not the ideal pp ab rehab exericse.
Women with diastasis should always avoid ALL abdominal exercises that lift/roll the upper body off the floor, or against the force on gravity as these types of exercises will worsen the condition.
Most standard ab workouts are not appropriate for postpartum rehab.