The convexity parameter basically tells the preview engine how many intersections a ray of light would have with a part. A convex part has a convexity of 2 (a ray goes in, and then comes out, two intersections). A concave part, such as a bowl, has a convexity of 4 (a ray can go in and out through each wall, total of four intersections). The letter E would have a convexity of 6 (a ray going vertically down through it would intersect six times going through each leg of the E). If you extrude a concave shape, the default convexity is 2, and you'll get these visual artifacts unless you set the convexity to 4 or higher. For efficiency, the OpenSCAD preview uses a convexity of 2 by default.
This affects only preview. If you render the part for saving as an STL file, the rendering engine doesn't need the convexity parameter and the render doesn't have those visual artifacts. You can see this when you render the part being discussed here; the artifact disappears.
However, you are correct that the code for this part doesn't have any statements that take a convexity parameter. So the preview engine is going to show this. The bug isn't the fact that this artifact appears (that is expected, by design). The bug is that there is no option in any of the statements to get rid of it for this particular case.