He knows customer support, not technical support. Customer support doesn't need to know anything technical, when you call them or contact them, they type your problem into a search engine and respond with what the computer says. They aren't versed in what card performs like what, what card is a rebrand of what, etc. Most of them are normal people pulled off the street, given a few days of training(on how to work the phone system mostely, and how to search for problems in the search engine) then tossed out into the call center.
They always make the first offer for replacement based on value, not performance. Saddly, the 290X wasn't valued nearly as high as the 390/390X because AMD overpriced the 390/390X. In fact, Newegg routinely has 290X cards pop up refurbished for $220.(That's how I got mine.) The 390 is still $300, and the 390X(the card equivalent to his) is $400. But value wise, the 380X is the closest(the Strix version selling for $230). But the fact is they are at least offering him something, it isn't a huge rip off, value wise it is only slightly below what his original card is worth. They also offered to replace his card with the exact card he had, but it is going to take longer because they have to find one. These are all good things. It shows they are willing to help him to either get a quick solution or a longer solution that gets him exactly what he sent in. The 380X is also only their first offer, and it is basically the offer I would expect any company to make. Companies offer replacements based on value. However, you can respond and explain that the 380X isn't the same performance as the 290X, and the 390X would be the equivalent card you'd want as a replacement. They very well might say ok.