This is a common misconception. Scalable does not mean infinitely scalable. Not all svg renderers and files are created equal
What vector graphics bring to the table is rasterizing on demand. This means that the application showing them can redo the art. This is wonderfull at big sizes but requires special attention when images are small. This is why fonts are "hintted", in essence the font has bitmap versions for small sizes. Truth is at small scales your image needs to be designed to hit the pixels.
Second theres no particular guarantee that the renderer, that takes the vector data and makes bitmaps, does a good job. Each app does a different result, but here quite clearly the aa engine is antialiasing to black, which is wrong. This is quite typical with engines that do not supersample the graphic.