AFAIK background is always treated by Corona as being at infinity and is bluring accordingly (physically correct). If you didn't change film width, then the other factor that controls DOF besides apperture is focus distance. Set your focus distance farther from camera and background will come more into focus.
If you want the background to be not totally blurry at - let´s say F-stop 4... your focus must be extremely far away... in Corona.
But what if I don't want a focus point that's far away?
Then the blur is so extreme that the background (override) is totally washed out...
In reality it would be blurry... but not that strong.
EDIT:
I percept the sense of Direct Visibility Override for placing a background pic like a garden, a landscape, etc... without using a plane or cylinder in the scene itself.
Gardens and landscapes are normally not "infinitely" far away... :)
As you said: the problem is that Corona treats the background override as "infinitely" far away.
In real world, the horizon is not infinitely far away. No photo camera will produce such an effect.
I was photographing a couple of years fully manually and understand the basics of this.