Here are current photos of the Biltmore Estate that I was able to find:
In this photo, the forest in the background of the painting looks very realistic. I now understand why people did not associate Olmsted's artificial cultivation with this "wild-looking" forests.
Likewise, the forest shown here in the background reveals a mix of different trees. Because there are many different types of vegetation it looks like an untouched forest.
The pond shown here is meant the give the estate a more natural vibe with its floating leaves and murky water.