Ummm, I look at this differently. If you think of what many will call "classic" tennis, then the player would step on to the front foot, rotate their core and make contact a few inches in front of their front foot. In this video, both players are rotating on their back foot but they are still making contact a few inches in front of their body and they are still rotating their core. You can actually hit with an open or semi-open stance and pull the front foot backwards as you rotate your hips and shoulders into contact. The key point is loading the back leg and rotating the core forward AND making contact a few inches in front of your body. If you doing all those things, you can still hit well and it is a necessary skill to handle fast deep balls. Graff hit a lot of her forehands with the front foot off the ground as she rotated on her back foot. But again, she got core rotation which moved her weight into the shot and her contact point was out in front of her body. Ideally, it would be good to transfer weight to the front foot as you step forward on to the front foot but it is not always possible. A good drill is to take a few shadow strokes in neutral, semi-open and closed stances and get a feel for your contact spot or contact zone. The contact zone is likely about 6 inches in front of your most forward foot. Then practice and try to "defend your contact zone" by never making contact later than the correct zone. It doesn't matter if you are stepping on to the front foot and then rotating or rotating off the back foot as long as your contact is out front in the zone and you are rotating your core.