"Some men are longer than others."
I agree with your first sentence, but I'm not sure I completely follow you on the rest.
I think there's a misunderstanding, possibly on my end, what a "terrain editor" is by definition. From my perspective, it's a manipulation of the core geometry which was not a part of the original Forge implementation. For example, you can't delete High Ground's ground or base because it's the core geometry of that level. You can edit objects by creating, deleting or removing them -- just not the core geometry.
That being said, if the video we saw is accurate, then it is a terrain editor, because the terrain is no longer the default Sandbox core geometry. Now how far down the rabbit hole Sandbox goes, there's really no way to tell at this point. I do not believe you're going to have different texture skins for landscape or that you're going to be able to craft, one-by-one, dunes and other elements which have, up till now, been core geometry.
I do believe that there's got to be some scalable measure of creative influence between the blue grid we saw in the video and the blue grid we see here. How much that is, in fact, remains to be seen. Even if Sandbox is just all that we've seen so far, I don't think there's any arguing that its potential has just engulfed Foundry like plankton in the mouth of a whale. I remain enamored.