Well, you're basically correct. The course is very much a collection of holes transplanted onto a piece of farmland. You can see that on the drive in and from numerous high points on the property. When you play the Straits, you're in an island of faux dunes surrounded by cornfields.
But it's not a sandy and scrubby site. It's almost entirely engineered, and while Dye brought in a lot of sand to fill bunkers and manufacture some movement in the terrain, it doesn't make up for the fact that it's ultimately a links-looking course built on a site with pretty heavy soils. And while the fairways are fescue, it's been discussed here many times that fescue doesn't thrive in conditions that are both warm AND humid, as Wisconsin summers are, and particularly when seeded in soil that isn't especially sandy.
So yeah, it would be great if the course was playing browned out and lightning fast, but it's not especially realistic given the climate limitations. It's just very hard to take fescue to the edge on a site like the Straits without compromising turf sward quality. That said, I don't see much reason to complain about the firmness given the rain they got just yesterday. The course isn't as bouncy as Chambers Bay was, but it's not any softer than The Old Course at The Open.
Just curious, have you read Ran's profile of Whistling Straits under Courses By Country? It gives a good account of the property limitations that Dye faced while building the course and some idea of the engineering effort it took.