I would agree with you if the image processing algorithm were actually well thought out. As it is, I fear that this article is encouraging some very bad ways of thinking about problems.
First off, they don't mention searching for an off the shelf solution to the problem. If OpenCV's built-in GrabCut or Watershed filter wouldn't work for them, they should explain why.
Secondly, they don't examine the literature for existing approaches to the problem. Sometimes you won't find what you're looking for in those approaches, but in that case, the problem with existing approaches will inform how you decide to tackle the problem.
Finally, they solve the problem by building a filter chain, but they don't seem to actually understand what the filters are doing. They say that the Sobel operator "looks for light to dark transitions". This is completely incorrect. The Sobel operator does nothing more than estimate the magnitude of the gradient of the image. Furthermore, based on this misunderstanding, they invert the image before applying the Sobel filter - a step which does literally nothing.
