Water on Mars: Lake or Meteor?

I was reading in Scientific American that NASA scientists believe they
have found the sight of an ancient lake on the surface of mars in an
area called Meridiani Plains, based on the large concentration of
Hematite on the area. But couldn't the water that formed the Hematite
just as likely come from a Meteor or Asteroid or other small celestial
body compacting the surface of the planet. It just seems to me that the
guys at nasa are jumping to conclusions. Is there any reason to rule
out the impact of a meteor or comet as the source of the water taht
formed the Hematite?