I also wonder why they call 'sustainability' a 'statement.'
There has been a pretty big movement in reduce/reuse/recycle (i.e. The Green Movement) for quite some time. The article is essentially meaning that it is now fashionable to wear clothes that embrace the environment. So, clothes made from used water bottles are now becoming "in style" (popular) to wear for example. So, by wearing those clothes, you are also stating that you care about the environment.