Surprisingly, a tummy tuck, along with other kinds of skin removal surgeries, isn't very invasive at all. As my plastic surgeon pointed out, there isn't any cutting into organs, joints, or muscles. It's all superficial skin removal. It certainly can involve a lot of skin, but the recovery isn't as difficult as for surgery that removes organs or cuts into muscles and bones. So that's the good news.
I don't know the answer to your muscle question, though. I had two babies and most definitely needed to have that gap stitched up in order to ever have a flat stomach. If your ab muscles have never been stretched apart, it only makes sense that they don't need to be repaired, but that's just a guess!