I do not think what they say is that it is hard to visualise it, but that it does not offer much utility to do so. A "for" loop like that is not that complicated to understand and visualising it externally does not offer much. The examples the article gives is about more abstract and general overviews of higher level aspects of a codebase or system. Or to explain some concept that may be less intuitive or complicated. In general less about trying to be formal and rigorous, and more about being explanatory and auxiliary to the code itself.