Because depending on how you’re looking at it, it’s technically a 4k ultra wide monitor. It’s, by the numbers, 8k across but it’s effectively just 2 4k monitors in one. It’s not “real” 8k like most people mean when they say it, as in pixel density equal of 4x 3840×2160. A “true” 8k ultra wide would thus be like 15,360 x 4,320, or more depending on how “wide” you go.
Ultimately just understand terms like “8k” and “4k” and whatever are just marketing terms. Just focus on what the pixel count and density is, that’s what’s ultimately important.