I think someone must have misunderstood the article relating to eye shine. Most puppies have a blue cast, so forget about that, but if a Yorkie has a green eye shine, his coat will more likely be silky, if a Yorkie has a gold cast his coat will likely be more cottony. Now here's where you statement about dark vs. lightness comes in. A silky coat is not pure black, but the preferred color is a gun metal grey, or metallic black that they call dark blue. Of course, many silky coats are often lighter, there’s a third gene that is a dilution gene and it seems to go hand and hand with the silkier coat. The perfect silky, dark blue, straight hair it's a very difficult color to achieve and unfortunately some breeders don’t even try to breed for it. Cotton coats, on the other hand, don't have that metallic element, so they are more likely to be a truer black. So in essence your statement could likely be true, just because cotton coats are more likely to be solid black. One of my dogs has a silky coat, which is super easy to take care of and he has green eye shine and his coat is a dark gun metal black. His brother has more of a cotton coat mix and his eye shine sometime is golden sometimes green and his coat is slightly darker than Joeys, but still what I'd call a gun metal. This article explains eye shine and what causes it.
Eye Shine