From reading through the supplementary materials for the paper [0], it seems that the authors are aware of this flaw and they found a way to make it hydrophobic. The approach they tested involved soaking the ceramic in a bath with a commercially available fluorosilane [1] that is used to make things superhydrophobic. Fig. S20 in the supplementary materials has a chart that shows the treated ceramic being very good ("solar reflectivity...remains at ~99.0%") but not quite as stellar as the untreated ceramic.

[0] https://www.science.org/doi/full/10.1126/science.adi4725

[1] https://www.sigmaaldrich.com/US/en/product/aldrich/667420