Let's not over-state the conclusions to the science here. Yes, the data show that during sleep, the space between neurons becomes more accessible. They also showed that, when injected into the brain, small peptides like amyloid beta (the major culprit in Alzheimer's Disease) and insulin are cleared faster during sleep than during waking.
Those are pretty exciting results. However, most pop-media mistakenly and automatically make the unfounded (but admittedly reasonable) leap that this clearance is therefore proven to be one of the primary purposes of sleep. This was not directly demonstrated by any of the findings of the study. Indeed, that this mechanism, under normal circumstances, provides any kind of 'washing' of toxins is not at all clear. After all, "good" signals would probably get washed away ,too.