Instead of an add-on, they turned it into the PC-FX.
Or maybe HuVideo. 20,000 frames and 60 minutes is only 5.5 fps (figure 33 minutes at 10fps), and I assume that's the max amount of frames, leaving no room for code, game graphics, or audio. Cut down the window size to save space, leaving room for tunes and the actual game, and that doesn't sound all that different than what's in Gulliver Boy.
Or perhaps the answer is
both. The specs don't sound all that impressive, not enough to warrant the cost of a presumably pricey add on anyway, so maybe they developed HuVideo to get as close as they could with existing hardware and let the rest wait for the better capabilities of the PC-FX.