It is not necessarily a comet if it were from the impactor (though most of the times it is a comet) because asteroids called chondrites have water, and chondrites have high mass (Approx 1000 Kg Chondrite found at Earth in the Jilin meteorite shower) because chondrites have iron in them 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 as it is more volatile than iron (1,538 °C), nickel (1,455 °C), copper (1,085 °C) and rain as snow which would not cause the impact and seismic activity and if it were a comet the ice would not cling to the surface of the crater but would rain from the cumulonimbus cloud and be a bit more spaced, because the comet would start regelation and vaporize due to the collision of air molecules in front of the comet creating pressure.