Does a 10H ceramic coating mean my car won't get scratched?

No. A hardness figure such as 10H or 9H comes from pencil-hardness testing and describes how a cured coating sample resists a pencil lead under specific test conditions. On a car, the coating is a very thin layer, so it can still pick up fine scratches and swirl marks from dirty wash media, deeper scratches from keys or grit, and chips from stones. Hardness also is not a direct measure of durability. A hard coating helps the surface resist light wash scratches better than bare clear coat, but it does not make paint scratch-proof.