Oh my. Consider for a millisecond why I used the terms "sensor pixel" and "image pixel" in my post. The difference is important and clearly the difference escaped you, leading you to make yet another false assumption and yet another hilarious conclusion.
I said: "A Foveon sensor requires one sensor pixel for each image pixel, because a Foveon sensor can detect all three colors at each pixel." Your assumption that a sensor pixel equates to a photodiode is utterly false. In this useage, a sensor pixel equates to a photosite. Each sensor pixel/photosite in a Foveon sensor is a stack of three photodiodes, with each photodiode responding to a different photo wavelength. Thus each pixel/photosite in a Foveon sensor responds to all three photo wavelengths. Or put another way, a Foveon sensor requires one sensor pixel to generate one image pixel (color).
In a Bayer sensor, each sensor pixel/photosite only responds to one of three photo wavelengths. Thus a Bayer sensor requires three sensor pixels to generate one image pixel (color).
In a monochrome system each sensor pixel/photosite results in an image pixel. Thus a monochrome sensor requires one sensor pixel to generate one image pixel.