You really need to ask KMC if they were using the term "stretch" in the literal sense or not. Advertisers being advertisers, they might have been using it in the literal sense. It's not their fault if people didn't realize that chains don't stretch and they weren't talking about actual chain wear, which is not stretch at all though we like to use that term.
Essentially they are just conveying the idea to you that this chain might take longer to wear than another. How much longer? Probably not enough to matter one way or the other. Get the chain that is the correct type/size for your bike, and the price, color/finish that you like and call it done.