It is not necessarily a comet if the ice / snow was from the impactor, because asteroids called chondrites have water and chondrites have high mass (approx 1000 kg chondrite found at earth because chondrites have iron in the Jilin meteorite shower) but there is no proof that it was a chondrite.
So the Martian atmosphere (100 times thinner than Earth's) still could cause a comet to evaporate (because the comet would start regelation and vaporize due to the collision of air molecules in front of the comet creating pressure. We can calculate the amount of heat generated by the ideal gas law.) as it is more volatile than iron (2,862 °C), nickel (2,730 °C), copper (2,562 °C) and will rain as snow which would not cause the impact and seismic activity. If it was a comet the ice would not cling to the surface of the crater but would rain from the cloud and be a bit more spaced.