That's because they used the X or Y Axis of the terrain to paint. If you have a steep cliff, textures usually get blurred liked that - in that case you can change the axis. If someone however changes this steep cliff to something flat like you did, it gets blurred.
To get rid of it, go to your texture painting tool (and sorry, I haven't opened Redkit in many weeks, don't remember the exact names) and somewhere above where you can set the radius of your brush, there should be something like "default" or "none". Change that now to something else and paint all over this terrain first (it will most likely all get blurry). Now you can change back to default again and paint it. The blurry parts should disappear.
