If diastasis recti is fixed through therapy, would a tummy tuck still be required?

Hello and thank you for posting your question. The answer is that therapy will not be able to correct the separation that has occurred between your rectus (abdominal) muscles. Instead of spending money on what I assume is physical therapy to try to "tighten" your abdominal muscles, you should save your money for the gold standard treatment which is an abdominoplasty (tummy tuck) with surgical plication (bringing together) of your separated rectus muscles. 
Plastic surgeons have a vast experience performing plication of the rectus muscles as it is routinely done during almost every tummy tuck procedure performed. I specifically perform repair of the diastasis during each of my tummy tuck surgeries. Patients are very happy after their muscles have been brought back together and no longer have a bulging abdomen. Hope I could help answer your question.
All the best!
Alex Zuriarrain MD, Plastic Surgery 

Therapy will not be able to correct a diastasis, but can improve the tone in other areas of the abdominal wall to improve your contour.

There is no other treatment for diastasis recti repair other than surgery (don't waste your money on therapy).  The muscles are laterally displaced and therapy can't spontaneously realign the muscles.  You would still require excision of the loose skin and it is very likely that the cost would be the same for a TT - since muscle repair is included in this procedure.