My possible explanation - With me not being a physicist I don't understand all the details about space and gravity but the only thing i can think of is that if they stay on the same spot they will have to constantly use fuel to stop the Soyuz from floating away and keep them selves in the same spot, better to just use fuel once to put them selves in an orbit that will collide with Muttas and adjust once or twice to match Mutta at the meeting point.
What do you think??
From what I understand, they both try to make a full orbit and intercept at the same location. In reality, this is only possible if Mutta drifted away perfectly parallel to the moon's surface. Unrealistic, but it's Fiction anyway.
Now I don't remember if they said what orbit they are in, so I will just do some guessing.
Assuming a 100km orbit, one orbit would take around 2 hours. So plenty of time.
In terms of speed, Mutta would intercept at exactly the same speed, he was overshooting the Soyuz.
So they would only need to boost in his direction a bit right before he arrives.
Since they don't have his exact location, the best thing they can do is wait for him to come back and try to catch him.