Radio Waves / Light: Atmospheric Refraction

The following image differentiates between a visual horizon and a radar horizon.

Sidenote: I'm not too familiar with primary surveillance radar technology, but this image refers to secondary surveillance radar, where the aircraft emits a radio signal at 1090MHz. So this refers to a simple one way radio transmission from the plane to the antenna.

The source claims the radio horizon is extended due to atmospheric duction, which causes electromagnetic radio waves to be refracted towards the ground. Snell's law suggests the refraction can be described using only the velocity:

$$\frac{\sin \theta_{1}}{\sin \theta_{2}} = \frac{v_1}{v_2}$$

To my understanding electromagnetic radiation and light are the same thing? Radio signals simply have a different frequency than visual light, but both have the same velocity. So shouldn't the visual light be refracted exactly like the radio waves are? Shouldn't the visual horizon be equal to the radio horizon?