Do I think it's a logical suspect? Of course, it's too obvious a possibility not to be a real possibility. You seem to even acknowledge that.
On the contrary, I think it's complete nonsense. Why would he use Mars
as an exemplar for an Earth
like planet? Even back before Mariner, even in the days of Percival Lowell and H.G. Wells and Ray Bradbury, it was always assumed that Mars was at most borderline-Earthlike, a dry, dying world on the edge of habitability. The idea of using it as the exemplar for worlds that are suitable for human life makes about as much sense as using Neelix as the exemplar for gourmet cuisine. There's just no way in hell that that could possibly have been Roddenberry's thinking.