What preview button?
You are definitely not crazy for suggesting that there is ambiguity on that space. We argued about it all weekend long this past weekend. Further evidence for hills: there is another region (on the four player map, I think, but it may be on the five) that also has an interesting little obelisk in it as well (west of, but near the center lake, IIRC) and that region is a little more clearly a hills region. I also vote for hills.
All green regions without trees are hills.
I'm not entirely convinced there aren't three or four little trees in there, hence the ambiguity.