It's explain in some detail in the article, but in essence, imagine a fine pen line which in 540p would be less than one pixel wide but in 2160p would be multip

It's explain in some detail in the article, but in essence, imagine a fine pen line which in 540p would be less than one pixel wide but in 2160p would be multiple pixels wide. The problem solved by Anime4K algorithm is essentially producing sharp edges of the line when upscaled to 4k, which is a different problem from upscaling a <1 pixel antialiased line.