I just saw these nebulaes earlier today.. They just lack one important thing: My ship flying between them... FDev, please hire that guy and bring to us these nebulaes..[yesnod]
From the vimeo page for this video, the creator replied in the comments to a similar question:
"Thanks! In Houdini I use particles controlled by physics and noise maps. Then I will render in Houdini the nebula form in slices from front to back, place them behind each other in afx and render and animate the camera shots. Hope this helps." -- Teun van der Zalm
I think, it means they're layers of two dimensional particle renders. 3D is normally rendered like that as it saves a lot of time/performance.
Looks awesome, though! I hope we'll one day get the ability to super cruise around nebulae in-game. At the moment the skybox is a static rendered 360 degree image.