Firstly, they don't contend he was murdered per se, but rather the victim of manslaughter by two guys who were basically playing with a gun. Second, they say that van Gogh accepted responsibility in order to protect the boys from further prosecution, which makes the choice of "murder" even more extreme. Thirdly, experts are as yet unconvinced, making the flat statement in the headline somewhat hyperbolic.
The BBC article is much shorter if you'd like a quick overview: http://www.bbc.co.uk/news/entertainment-arts-15328583