I can give you a glimpse into the past. Amiga had a very slow CPU (7 MHz) and each instruction took between 4 and 40-ish cycles. However, it had 3 special purpose co-processors. For these demos, Blitter (very fast bit copy and memory fill functionality) and Copper (change palette and even resolution on a specific vertical scan location) were essential.
The technique is the same one we used to make 3D games - you would draw an outline (taking care to write a exactly single pixel on boundaries, your line-drawing routines had to be written to take that into account) and let Blitter fill this. Amiga had 6 overlay planes you could use for up to 64 colors (well, it was actually up to 4096 colors with caveats, but let's ignore that for a moment). So, if you write a filled outlines (time shifted) in those 6 planes, you'd get this nice blurred effect.
My guess is they manually wrote pixel coordinates for the images, connected them with lines, filled with Blitter, repeated for each frame. It's how things were done back then. Without Internet we had a lot of free time (I was a kid in high school).
Those special purpose chips were magical. At the time of IBM XT and its green on black terminals, Amiga was a space shuttle in comparison. For example, my game used to have upper part of the screen in lower resolution (where we painted vector graphics), lower part in higher (where we had game stats).
> "State of the Art was traced by hand with a Genlock overlay and tool I developed. In 9 fingers the process was automatic, my program controlled the videoplayer, digitized one picture, traced it, and skipped to the next frame. For me the equipment at that time was expensive, about 150 Euros for the videoplayer (used the prize money from State of the Art), since it had to show de-interlaced pictures."
