This is what I think is really the reason people gift and wear engagement rings: upholding tradition and signalling commitment. Even if the tradition is "fake", created by De Beers marketing, isn't most tradition like that anyway? Artificial gemstones don't lie too well with people who care about the tradition of gemstones. I don't even think it's necessary to care for the tradition, I think the value of natural diamonds is pretty much burned into everybody's subconscious by pop culture. And the inflated cost is actually a "plus" for signalling commitment.
Don't get me wrong, I would feel pretty stupid if I had to buy diamond jewelry (if I could); I just don't think the article makes a good pitch, because its points don't hit close enough to the reasons people buy (or desire) diamonds. I kind of think that the article speaks best to those who are least inclined to buy diamonds anyway.