You should always start from source IMO.
> Blender exports KTX2, your engine supports KTX2, Gimp exports DDS/KTX2, Substance Painter exports DDS/KTX2, three.js has a KTX2 converter.
If you're manually doing this conversion from source images to shipping formats your wasting your artist's time, AND, you'll likely lose the source images since people will generally only give you what's needed to build. "Hey, could you tweak building-wall-12.ktx?" "No, It was made in Photoshop and I can't find the file with the 60 layers so no, I can't tweak. Sorry"
In this case, the source are TIFF's, pulled from the company texture CDN... I agree you start with high resolution sources but you don't ship lower resolution PNGs unless you're on an indie game or making a browser game.