That's like saying "You can Convert your Gold Bar in Dollars and shop on Amazon so that doesn't change the utility of Gold" . This is wrong. If tomorrow my convenience store accepts Gold coins, of course it increases its utility because it removes time, effort, and additional steps (i.e. barriers) to use it.
Yeah the utility would increase, but I think his point was that it wouldn't "explode".
I think it's more accurate to say that Amazon accepting BTC would place a minimum value on BTC, assuming Amazon didn't peg the price of items in BTC to the price some other currency, for example if they offered to sell the Nexus 7 for 0.5 BTC, then 1 BTC would be worth at least USD600.
The reason why I said it would explode is because right now Bitcoin has virtually no utility on the market, apart from niche usage (HB, Namecheap...).
As for the value of BTC, nothing would prevent Amazon to have a fixed price in USD and a floating/real-time adjusted price in BTC (that's what merchants using BTC do anyway currently).
Maybe the media exposure would be great if Amazon accepted it directly but it wouldn't really change the utility of Bitcoin.