They mentioned it here somewhere in the thread and also on their discord (including some preview images about the celestial addon on discord).
The PSA clouds looks more like painted, but not real, because they miss volume.
VDB clouds are real 3D objects, but slow to render and you need expensive and complicated software (like Houdini) to generate them or you buy some VDB clouds. While you can’t create them in Blender, you can easily use pre-made VDB clouds.
Just compare these two images. In the first one I just use the clouds from PSA:
and in the second one I use only the sky and sun from PSA, but clouds are VDB clouds