Here's direct quote from the original published paper that acknowledged that and kinda makes it seem like the authors of OP's article hammed it up a bit:

"The device is tested in Phoenix for 37 h, and the chamber with the TRC can have a temperature up to 6.1 °C lower than that with the glass slide (Figure 4B and Figure S7). We note that both chambers have temperatures higher than ambient temperature because the transparency in the visible wavelength range of the TRC allows solar heating. In practice, TRC can work with other existing (e.g., air conditioning) or emerging cooling technologies (e.g., subambient radiative cooler) to reduce the overall cooling energy consumption of buildings."

They are, but due to scattering and refracting in a normal, unprotected room, less surface area is exposed that could absorb the heat. So curtains cool rooms pretty well. If you want to increase that, you could make reflective curtains I guess. But your neighbors wouldn't be happy, and it'd look weird.

Yep, it's referred to as "curtains" when I use it though.