Defishing is a process for removing barrel distortions in fisheye images. It takes fisheye images, and convert them to normal, rectilinear images that preserve straight lines. Different fisheye lenses can have a different amount of distortions, and thus, require a different defishing strength.
Some flat-Earthers would deliberately use stronger defishing parameters than necessary for the lens being used. Their intention is not to remove the barrel distortion in the resulting images, but also to introduce pincushion distortions to the images. Pincushion distortion is the reverse of barrel distortion. It would curve a straight line to the other direction from what would be produced by the barrel distortion in fisheye lenses. At a specific position in the scene, it would make the curved horizon to appear flat.