I think what the article is saying is that you should have one set of targeted key words per item, and don't try to add the same key words to every item you have, b/c that will water down your results, so to speak.
So, if you sell small plaid hair bows, and large red hair bows, you shouldn't try to keyword both listings with hair accessories, b/c that's much less specific. Instead, you could target the small plaid hair bows with "small plaid bows" or "small hair bows" ... and the larger ones as "large red hair bows" or "large hair bows" or "red hair bows".
I think the basic idea is to be really specific, b/c the more you generalize, the less usefull your keywords become. If you target for "hair accessories" , someone might find you like that, but they might not be looking for bows. They might be looking for hair ties, or tiaras, or fascinators. In other words, your SEO efforts are wasted. But, if someone searches for "small plaid bow" and they find your listing, it's exactly what they were looking for, and they're much more likely to actually buy your product.
By using different sets of key words on different listings, you have an opportunity to get more views to your shop, from buyers who are looking for exactly what you offer.
Does that make sense? I hope it does, it made sense in my mind! Hope it helps!!