You're assuming that Foveon sensors are perfect also. They are not. There's a lot of crosstalk between Red, Green and Blue. The blue channel contains large red and green components. Green contains a lot of red etc. It's this crosstalk, plus manufacturing variations, that cause the colour inaccuracies that I was talking about.
Modern CFAs deliberately introduce some crosstalk between adjacent colours themselves, so each photodiode gives more spacial information, plus the filters are formed into lenses to increase the amount of light hitting each photodiode. The result is higher light sensitivity and a more predictable colour response than Foveon types.
Use your Sigma and be happy, just remember that there are reasons that most of the photographic world uses CFAs and are happy with them. (And no, I don't work for a CFA manufacturer, nor any photographic company. Nor do I employ other accounts for down/up voting)