No I have no affiliation - found the website after reading BBC News (http://www.bbc.co.uk/news/technology-19813382).
http://defensedistributed.com/products/ specifically talks about manufacturing an AR-15.
Whether they are commercial / advocacy / non-profit seems irrelevant. If they publish information relevant to wiki topic then can it not be used a reference?..
If a third party article described a method of milling the lower receiver of an AR-15 at home then it too could be referenced.

The project's key focus appears to be circumventing legislation and they use the AR-15 as a specific example. The Wikipedia page lists the manufacturers and then goes on to detail, at length, the global legislation controlling the distribution of the AR-15. Both these sub heading seem directly effected and relevant to the referenced article.