I thought this sounded preposterous at first. But since Yamaha makes very good musical instruments, I tend to respect them. Just speculation, but if these speaker cabinets are fabricated with wood, the paint finish might be some sort of epoxy or high strength material, that stiffens or damps the wood. Being a DIY woodworker, I have experienced that some of my painted projects seem to take on a better structural situation after painting. Just a thought!
Unbelievable
Yamaha really made this statement:
Glossy black piano finish provides improved signal-to-noise performance
https://europe.yamaha.com/en/products/audio_visual/speaker_systems/ns-5000/index.html
I thought I would seek opportunity to hear these speakers, but now I do not think so