This is an argument that I've read often, but I've also seen little to support it other than that this is the way that the person making the comment sees things. That's all right. But the argument may at best be nothing more than a truism. I would argue (but can't prove, of course) that there are more people on the face of this Earth who love Prof. Tolkien's maps than there are those who love all of the maps of all of us here at the guild combined. And I personally doubt that more than a minuscule number of the Tolkien map lovers ever even has thought about whether the Mountains of Mordor make sense. So ... yes ... I see things more the way that LadieStorm does.
That, on the other hand, I consider to be a good guideline but also something less than a rule. When one makes things that are unreal, it can baffle the person being addressed, because it doesn't make sense. There are some things that might need to be done then to address this unreality. But it is not always necessary. One's fantasy world can have something like the Mountains of Mordor in it, because if one's point is to illustrate a story, as Tolkien's was, only a minuscule number of the readers will notice that such mountains are unreal and many of them would need an explanation of why they are unreal, because they lack such knowledge of mountains. For them, Tolkien's unrealities by no means shatter the suspension of disbelief. Nor do rivers that split, for that matter.I think a fairly good maxim for helping achieve that balance would be: Make things as they would be in reality, unless you intentionally mean them not to be.
However, there is a point when one can use unreality in such a way that it will shatter the suspension of disbelief of almost anyone that encounters the map. There are few maps that can have all of the rivers flowing uphill and across mountains without providing the user with a lot of help. In that case, to avoid shatter(ing) the suspension of disbelief, the map maker and author of the work or RPG game needs to create a virtual reality for their own world. They need to do more than to merely say that things are different in their world or to cop out by saying abracadabra, magic! They need to define in an understandable and credible manner why things are different within their virtual reality than they are in our reality and how those things can work.