We don't have all the context, because we're only seeing one page. Nevertheless, it seems like this isn't just oversimplification because "never occurs" is not a really good simplification of "usually occurs much more slowly in solids compared to liquids". It is noteworthy that this is not the only error (or simplification, if you prefer) in the image. For example, diffusion does not always lead to uniform concentration everywhere, even at quasi-infinite time ("their particles are evenly spread - their concentration is the same throughout"). E.g., a substance dissolved in one medium diffusing into another medium, will not have an equivalent concentration across both media, because of different solubilities in each medium.