IIRC Nebula has an exclusivity clause in their contract not allowing content creators to upload to other platforms, though I could be thinking of CuriosityStream.
You must mean CuriosityStream because most of my favourite Nebula creators are also YouTubers. Usually, the Nebula version of their video is slightly longer, too.
This point was raised in the video as well, that Nebula has an exclusivity clause. Occasionally, there is a Nebula-exclusive upload, but my experience is the same as yours; most videos are also uploaded to YouTube (usually with a sponsored ad inserted).