Barium sulfate based coatings have been used for decades, maybe a century, as the 'whitest' reflecting standard for optical instruments. I hate to be that guy, but there's nothing that new here. Though Purdue and this prof sure are hyping the heck out of this, it's pretty funny for anyone in the optics world.. we buy it all the time for integrating spheres! This wasn't published in a high profile academic journal for a reason...

The actual challenge with any solar reflective paint is that it loses its solar reflectivity after multiple years of outdoor exposure. This can be seen at the third party Cool Roof Rating Council's website: https://coolroofs.org/directory

Outdoor white paints that start at 90+ % solar reflectance drop to the low 80s with time. Same thing will happen with this one, so the added benefit of the high reflectance will decrease quickly. Figuring out how to maintain this high reflectance in a paint exposed outdoor remains a very hard problem.