I had a chat with Cliff directly about this but for the benefit of others with a similar concern I will repeat here. As the garage door runs on bottom mounted rollers we cannot add anything to the underside of the door to close the gap. We also cannot modify the door due to the safety edges.
There is a solution but it would require building up the floor to be closer to the garage door. I think this would resolve it, but it is not a cheap option as there would be a number of design considerations to factor in. It would probably be easier if the slope down to the garage was less steep.
Thus it will remain an annoying nuisance 🙁