Most high-res texture packs for block games I've seen work by grabbing random slightly related photorealistic textures and slapping them onto blocky voxels, which always give a bit of jank along with ruining the look and feel. This texture pack manages to not do this, by carefully making the new textures be as faithful to the original. They're still high-res and photorealistic, but they do not look or feel out of place. Sometimes, they look totally like the original texture, except upscaled and with a texture to it, as if you can reach in and touch the texture of the grass (heh), or feel the bark of the logs.
In addition, there's also hints of pixelation in e.g. the grass node side texture, leaves or the top log texture. Which just furthers the brilliant atmosphere of a high-res texture pack that is able to work with the fact it is still a voxel game, made out of cubes. It doesn't try to be anything more, and accomplishes very well because of that.
Honestly, I didn't investigate if they were copyrighted or not, because I didn't license the mod repository, I did license the mod in ContentDB, since I didn't know what I was doing.
But hey, I already changed the license of the mod in ContentDB to "Unlicensed", with the intention of letting it be known that I really do not own the images used to make this texture pack. The only thought I have is to adapt the images so that they look consistent (that they look mosaic) and with appropriate colors.
And I earn 0 dollars with this mod, so there shouldn't be any problem (I think).
the "Unlicense" option does not mean the content is unlicensed, it means that it's licensed under the terms of the license named the Unlicense. Please direct complaints about the confusingly named license to its creators.
Content for which you do not have the right to grant permission to use cannot be distributed via CDB at all. You will either need to track down the sources and get proper permission from them (in the form of a license, not just a personal promise) or you will have to replace the items for which license is not available.