This does fit very well with what we've learned about mars so far; A brief window of borderline habitability followed by increasing salinity, acidy and paucity of water followed by the hyper arctic desert conditions we see today.
But.. we've found life on earth in conditions of utter cold, hyper salinity and acidity. For microbes surviving in arctic ice, or the mcmurdo valleys pretty much the only water available is very salty or acidic, and very scarce in liquid form. If there ever was life there I dont think it's unthinkable that some hardy microbe found a way to survive, even as conditions turned very bad indeed. And lets not forget that there are places in the outer solar system that show signs of being much more habitable than mars ever was, even today. So I don't see why this should be taken as too great a disapointment, i'm going to take it as an object lesson in keeping what I hope for realistic, and carry on hopeing!