On Earth the horizon is just a few miles or kilometers away on a "flat"s surface, which is actually the slightly curved surface of a sphere thousands of miles wide. And Middle-earth is supposed to be our Earth about 6,000 years ago.
But suppose that geologic forces or the will of Eru bulldozed a small really flat area of the plains of Rohan, so they really were on a plane surface, a chord cut out of the usually spherical surface of the Earth.
Or maybe there was a very shallow crater about ten leagues wide, and the riders of Rohan were near the lowest part in the center and Legolas and friends were right below the crater rim. They could look down several hundred feet at the riders fifteen miles away who might not be over the horizon.
Or perhaps there was a very wide and shallow river/stream valley tens of miles wide and a few hundred feet deep. If the slope was even all the way down it would be about a one degree slope or so, which would seem rather flat. Would that elevation difference be stronger than the Earth's curvature?
Thaddeus wrote that on a level surface an average adult human can see to the horizon which is about 4.7 kilometers or 2.9 miles away.
I can tell you that from the boardwalk at Cape May, New Jersey, the lights of Lewes, Delaware can been plainly seen at night. So standing perhaps twice as far as eye level about sea level, I could see lights nearly 15 miles or 23 kilometers away without them being cut off by the curvature of the Earth.
And in the daytime, I have often aimed the pay binoculars on the boardwalk in the direction of Lewes and plainly seen houses, trees, and hills on the farther shore. Even without binoculars, I could just barely see a few tiny permanent bumps on the horizon in the direction of Lewes. Of course the height of the tide might affect how much of Lewes is visible at any particular time.
The elevation of Cape May is given as 10 feet or 3 meters, and its highest point at the intersection of Washington and Jackson Streets, is given as 14 feet or 4.3 meters.
For him to be able to see about 20 miles (given the curvature of the Earth) he would need to be 90 feet in the air. (REF: How Far Away is the Horizon; Discover Online)
If the ground sloped gently down about 100 feet in 15 miles and perfectly straight or even a little concave that would be about a tenth of a percent slope which should seem perfectly flat. Of course Legolas would have to be really "eagle-eyed" to tell the color of men's hair at that distance - several times as much as an eagle, in fact.
I wonder if Tolkien sometimes forgot whether Middle-earth or Arda was supposed to be flat or round during the era of a particular story when he was proofreading.