Yes, that skier is definitely perfectly balanced on the skis. He's turning his legs but not turning his hips or shoulders just as Xela said.
On a groomer, try side-slipping down the hill with skis pointed left but hips/shoulders facing as straight down the hill as possible. Can you side-slip straight down that way, not heading off forward nor backward but just straight down, sideways? Work on doing this facing left, and then facing right. It's easier on hard snow and steepish pitches. It's very hard to do on soft snow when the pitch is almost flat.
Then work up to side-slipping down just like above, but pivot both skis together, parallel, to pointing straight down the fall line. Do this from both directions. Then try doing it 180 degrees, from skis facing left to skis facing right. Point em left, then straight down, then right. The whole time, keep your hips and shoulders facing downhill. Work on doing that smoothly, alternating from pointing left to right and back again, on hard snow, on blue pitches. Keep skis parallel the whole time. Keep trying!
Next, work on doing it without travelling back and forth across the trail. That's right, just travel straight down the hill while pivoting the skis left then right 180 degrees, with hips and shoulders facing down the hill. This is not easy. It's called a pivot slip. It's figuring out how not to travel left and right as you pivot that gets everyone hung up. The skier in that video can do this.
When you can do it on hard snow, on a blue groomer, work up to doing it on soft snow, on a similar blue groomer. Then on your fat powder skis do it off-piste on different pitches. That's what this guy is doing, or a modified version of it. Take a season or two (or three, or four) to learn to do this, depending on how many days you get to ski and where your current skill level is.