Diastasis recti surgical repair: are the muscles actually stitched together or is the linea alba sewn?

It is possible to close the diastasis recti either through an abdominoplasty, endoscopic assisted abdominoplasty or laparoscopic surgery.
The abdominoplasty approach is accessing the surface of the muscle fascia lata and stitches the fascia titre bringing the edges of the rectus muscles close together.
The endoscopic abdominoplasty can sometimes be carried out just through an access in the bellybutton, or can be done through an incision in the suprapubic region. It does not require skin reduction like a normal abdominoplasty but is only applicable when the skin elasticity is good.
The laparoscopic approach is carried out by general surgeons and can sometimes leave fullness in the central part of the abdomen in the midline, since the skin moves closer to the midline together with the muscles.
An abdominoplasty procedure or an endoscopic abdominoplasty procedure also redrape the skin.

Thanks for your question.  Theoretically the goal of repairing the diastasis, or separation, of the paired rectus muscles is to restore the normal anatomic position by repairing the fascia edges, essentially creating a new linea alba.  The reason diastasis occurs is because the linea alba, or midline connection of fascia, is very inelastic and once it stretches due to weight gain or pregnancy it does not resume its normal narrow connection between the muscle sheaths.  Once the muscles are brought back to midline, it is then possible to strengthen them again.  In practice, we often bring the muscles together but then tighten them additionally by stitching the rectus sheath (not really the muscles themselves) to improve the abdominal wall contour further, especially if the entire sheath has stretched to a significant degree.  This completely depends on the patient, their level of conditioning and whether they are at a normal weight.  It is best to discuss the exact approach with your surgeon and determine what might work best for you and what the tradeoffs of each approach might be.

There are two main surgical methods for. closing a diastasis, plication or Rives-Stoppa repair, and both aim to bring the rectus (six pack) muscles together in the middle. 
Plication is used for smaller diastasis, 3cm width or less. A stitch is placed from one edge of the stretched linea alba to the other and tightened, which causes the widened linea alba to fold inwards. The concern is that the stitches are placed in stretched weakened tissues which may give way, or the stitch may snap, and the diastasis come back.
Rives Stoppa repair is good for wider diastasis (4cm or more) or when there is lots of bulging. In this procedure the fibrous layer covering the rectus muscles is cut along the inside edge of the muscles. This cut edge of one side is stitched to the cut edge of the other side, which pulls the six pack muscles together, with a mesh behind the muscles to reinforce the repair. The original stretched linea alba lies behind the muscles and mesh. This is a much stronger and more reliable repair, but it is a bigger operation. 
Both procedures can be performed using a cosmetic Abdominoplasty (tummy tuck) approach.