All the inertia is in the rollers. The mass of the vehicle isn’t moving with relation to the rollers frame.
If the rollers’ bearings seized solid and the vehicle drove off the front, the tyres would spin and screech a bit and the driver would emergency stop to a halt.
On a set of driven rollers, if the driver presses the brake, thinking it’s the clutch, the car would be pushed off the rollers with rollers screeching on tyres and the car would stop dead in about 3 inches.
You don’t have to wear a seatbelt in a chassis dyno test.