Highly doubt. Tech demos still use same engine. They are “tech demo’s” after all. Some of the tech may not make it to the final game due to performance constraints but if they did it on video, it means it is working.
Now, there seems to be a great misunderstanding, just because Nanite does not support it, does not mean you can’t use traditional UE4 workflow. So they could be using traditional UE4 style snow, they could be modifying landscape in real time, they could be using voxel plugin for snow, they could be using their own plugins for snow.