"4K" has different meanings depending on the context. Yes it defines a certain resolution, if we're talking about buying a monitor. But when we're talking about CONTENT PRODUCTION, saying art is made "in 4K" implies that, for the given area on the screen the art will take up, its pixel density will be 1:1 with the pixels on the display. This is how these terms are used when discussing art, because it's the most practical and easy to interpret.
If I take a 4K wallpaper, and then slice it down the middle, it's still an image made for a 4K display, it's just covering half my display now instead. Functionally it is a "4K asset". Arguing otherwise is being needlessly pedantic.