It shows on the link you posted it being used with no base and the seatbelt over the lap, so i'd say it's safe... here is what it says....
The SnugRide 30 is designed for use as a rear-facing seat for children weighing from 4-30 pounds, so you can keep it configured in a safe rear-facing position.
Yes it is made where you dont need a base. You run the seat belt under the openings on the sides. Still make sure to use a seat belt that locks or use the locking clip that is on the underside of your carseat.
Make sure the seat belt is able to lock or use the locking clip, but yes, it's perfectly safe with that car seat.