The textures merely have more pixels, they don't have any more detail.

On my 1440p screen, the extra pixels make a huge difference making the game not look blurry.

Yeah, i say the "HD textures" are just smoothed out versions of the originals. Absolutely no more detail than it already has. in fact the originals look better

I found it made a significant difference for me on 1440p. When zooming things are a lot more detailed. Looks like the author ran all the PNG files through a Batch Macro/Action Job that enlarged them and possibly added a bit of Sharpen or Smart Sharpen effects to counter the blurring that happens from the upscale.

WaiTex actually uses this project to upscale the images. https://github.com/nagadomi/waifu2x
The mod scales Textures with Waifu2x hence the name.