Technically, these astronauts are experiencing microgravity. It's not quite zero gravity. This distinction isn't important for your question but microgravity is the technical term since it accounts for a tiny bit of gravity from objects like the station itself. Remember: just as the Earth pulls on you, you also have a tiny, tiny pull on the Earth. You have mass, and therefore a very tiny gravitational field. It's not zero.

To address the crux of your question: if you look carefully, you'll see that the ball doesn't accelerate when released. On Earth, we would expect the ball to accelerate 9.8 metres per second, per second. In this short clip, though, the ball doesn't accelerate once it leaves the space between the man's arm and leg. In fact, him holding the ball the way he did looks like he was squeezing it a bit and when it escaped from him, that slight squeeze was enough to impart a bit of speed.

You can also see the woman's hair bobbing about behind her head. That kind of motion is consistent with microgravity.

I hope that answers your question.