Sounds as if he thinks the NYXL are standard strings, but they too are a specialty set.
Stiffer, harder and a little crisper sounding than the regular nickel steel XL110s.
Given that the NYXLs are fairly bright, I'm surprised that there isn't more of a disparity between them and the flats.
I'm glad he pointed out the the D'Addario flatwounds are built to be a bit brighter than flats from other brands.
Good to know. I'll try flatwounds on my rockabilly guitar one day. maybe on the jazz box too.
It does seem as if the flatwound set sounds and plays a little different on the plain strings as well. That's unexpected.
But perhaps it's just that the plains in the flat set are standard steel, not the NYXL steel.