Did you try the advice on the page? Hold WASD and try every other key. On my 2014 macbook pro, even WASD by itself fails, so I'd be surprised if the Air is any different.
My MacBook Air can do 6 keys, but not any 6. They have to be horizontally disbursed. In particular tight triangles are impossible (AWS, WSE, etc.) and cannot be pressed at the same time. A fun site to use to see this for yourself is http://benzguo.com/bayan/ .
I do have an Air (same model as OP), and found that WASD didn't work.