Yes—distance through air is a major cause of this, not a specific focal length threshold. Atmospheric softness depends on how much air is between you and the subject, plus heat shimmer, turbulence, humidity, haze, and particulates. Long focal lengths make it more obvious because they magnify a small distant area, so any atmospheric blur is enlarged too.

There isn’t a fixed distance where it starts; it can appear surprisingly early if conditions are poor, and may be minimal much farther away in very stable air. A subject 1.7 km away can definitely show visible haze/softness.

Your camera may also be contributing. On a superzoom, the effective f-number usually rises as you zoom in, and small-sensor cameras are more affected by diffraction at higher f-numbers. That can reduce fine detail even with perfect focus and clear air.

So the likely answer is: both effects can be present. For distant telephoto shots, expect softness to increase with subject distance, unstable/warm air, and higher zoom settings. Shooting in cool, still air (often early morning), avoiding heat sources, and keeping expectations modest at extreme zoom can help.