Thanks Chris and René for your comments!
This is an older project from the v7beta period when I had no license and couldn't save projects.
So I build simple stuff that could easily be recreated.
I found some screenshots of the settings and the materials too but apparently from earlier stages, not the final render.
See attached file without the menger frame and camera animation.
I could not achieve manageable render times with any kind of radiosity, so the whole thing has solid/emissive materials which render quickly in Cheetah (2x2 quality).
The fog is done via fog tag, no special trick involved.
The tree is actually a simple egg-shaped polygon object with a material with a turbulence fed into alpha-channel transparency to create leaves or branches with a see-through effect.
Because there is no radiosity I used an emission gradient that gets brighter towards the top in order to fake shadows, which does not work as well as the branch effect or the horizon rendering.
With these tricks I could manage to render four million instances at 2560x1600 in about 45 seconds and do a longer animation.
At the time I considered it a failure but later I found myself enjoying the animation in spite of its obvious flaws and decided to share it.