You know that's probably meant as a joke, right? ;_;
In Euclidean geometry, spatial concepts of lines, planes, polygons, and solids are entirely conceptual in nature. Infinite points constitute lines/planes/etc. The corner can't be "shaved off by erosion" because the point that occupies the corner (no matter what the 'zoom level') can't be emulated in the real world.
If the creator meant it as "all real world squares" (or even those represented by computer graphics in the real world) then yes...there are rounded edges in the real world. :p If "all" includes mathematical squares then maybe not. ^_-
Well, it's just (albeit in a parody sense) contrasting the mathematical, numeric perfections vs the outside world's realities that sort of bend those formulas, theories, etc.