Joe,
I have had the same thought. You are probably, right-on, this "red" D. capensis really needs lots of light before it looks any different from an ordinarily colored plant. I would guess, too, that lots of people do as you suggest, and wind up believing the seed were producing normal colored plants. If only the use of, too much light, were more common, then those kinds of problems would happen much less often.
I know that it is possible for this not to come true from seed, but in my experience, it rarely happens. I've grown hundreds of these from seed and they've all been quite red under good conditions. Perhaps if the seed were an intentional cross with the normal colored forms, then it might show more variation.