This is covered in the film's original script. In short, she hadn't killed his momentum, as evidenced by the fact that he continued to float away after she let go:
The tether attached to Matt’s suit passes two feet away from her and Ryan grabs the tether. It slides through her closed fist, until
Her grip tightens and the tether STOPS.
RYAN (CONT’D): Gotcha!
Matt’s momentum is now pulling Ryan, dragging her loose from the parachute.
MATT: You have to let me go.
RYAN: What? No--
MATT: You have to.
RYAN: No!
MATT: Those ropes are too loose, I’m pulling you with me. You have to let me go or we both die--