scribus does not support all svg features that inskcape inkscape does.
this having been said, the tutorial you're following is imo only useful if you exactly know which cmyk colors you will to replace the rgb colors in the svg.
(as an example, for a logo created with inkscape and you have graphic charter to follow...)
from the screenshot you have posted, i have a hard time believing that you will want to do that with that specific image.
if all you want is a pdf automatically converted to cmyk for printing purposes, going through an high resolution png might be an ok workaround.