I think you're slightly misunderstanding the argument here. Clay Matthews might only go up to 405x7 in squat, but that's by choice. He is a highly-paid professional football player; pursuing a 600-pound deadlift is not in his best interest. That doesn't mean he couldn't do it, only that he is choosing to spend his time training in the manner most effective for his sport. Speaking of which...
No, wait...you're misunderstanding something here. This thread isn't saying everyone should pull 600, not by a long shot. The discussion is whether many/most able-bodied males (in their prime) have the capability to attain a 600 pound deadlift assuming that they have trained to do so.
Whether NFL or NBA or MLB players can pull 600 now is irrelevant. They aren't focusing on the deadlift, if they train it at all (most probably don't); I do believe that most/all of them could pull 600 with a year or two of focused training. I mean, I'm certainly NOT a "genetic freak" - if I was, I would be playing in the NFL - and I pulled 500 last July with about one year of semi-dedicated training (which wasn't even THAT crazy; hell, I haven't done any barbell squats or basically any assistance lifts other than some leg presses).
Honestly, although my opinion (already stated above) is that I'd peg this line at more like 500 rather than 600, I think the basic sentiments reflected in some of the first few replies...
...already covered it.
The fact that most people don't want or need to put in the time and effort to accomplish this is immaterial. We all already know that most people don't have any interest in strength training, and even among those that do, only some actually care about their 1-rep max deadlift, so the population of people who are actually pursuing this goal is winnowed quite substantially. But I do echo the opinions of several people above that a 600-pound deadlift (or at least a 500-pound deadlift) is available to many more people than only the "genetic freaks" of the world. I stand by my comment above that I am, without a doubt, nothing at all resembling a "genetic freak" - and I expect to move well into the 500's this year, in what amounts to my second full year of barbell training after a prolonged break spent running long-distance races!
I do want to address one other point - @BrickHead above mentioning that if you look around, there are lots of diminutive, uncoordinated people for whom a 500-plus deadlift is a pipe dream. Of course in their current state a 500-pound deadlift is worlds away; but most of these people only appear so diminutive and uncoordinated because they have never spent any substantial time or energy dedicated to athletic pursuits! If every kid climbed the monkey bars and jumped rope and played soccer as a child, then played some team sports in high school, and took up lifting weights at 18, there would be a lot fewer "diminutive, uncoordinated" people walking around.
Look, I understand that most people don't deadlift 500 pounds during their lifetime because they have other (better) things to do; this is a purely theoretical discussion of whether that lies within the realm of possibility for most people. Only some people will ever lift weights, and of them, as @T3hPwnisher is fond of pointing out, most people who undertake strength training will "fail" but my personal belief is that lack of effort and/or dedication and unwillingness to critically examine one's strengths and weaknesses are the biggest reasons for that.