It is supported by physical facts. In fact, it is the only explanation which fits all the physical facts. The other models have to *assume* that Mars once supported a much denser atmosphere in order to have had liquid water flowing on the surface, even though there is no evidence for it, and no explanation for how it could have been that way, or what happened to change the conditions.
While I may agree with you on this theory, it is just that. Only a theory.
Until we can perform actual experiments on the surface of Mars, it will be impossible to know the difference between computer models and reality.
The current "Global Warming" crap has taught me to experiment with the actual physical environment and learn about factual reality. Sadly, today's computer models have become as abstract as Aristotle's thought experiments.