I don't think that's the entirety of his basic sentiment, it's a two-part proposition. From what I can tell the gist of his statement was that mankind's effect on the earth has been detrimental - and that because that effect has been detrimental, every human being must die. It's one thing to equate human life with animal life, I can at least understand how one would come to that conclusion; and I wholeheartedly agree that to insert an 'ought' into the evolutionary process is suspect, but taking the ought at face value I find it bordering on the depraved.
He is positing as a moral good the destruction of the entire human race in order to pave the way for whatever improved homo superior will supplant us.