Thanks, that's a good read, but doesn't say some people aren't contagious. It's saying that some people spread it more, with most of the theories involving their actions (like singing loudly in a choir or attending a Zumba class), not unavoidable physical characteristics.
One paragraph mentions differences in their immune system or the distribution of virus receptors in their body as factors. Not a characteristic that determines whether or not they're contagious, but determines how efficiently they might spread it.
Everyone is contagious if they have it (for two weeks, IIRC). Most contagious at the beginning and less contagious toward the end.