I was given to believe that they were not considered canon. I thought that material submitted to any sort of support magazine is fan material and thus not canon (unless officially endorsed as canon by the game designers themselves)
However I think some articles in Challenge were written by people who were also contracted to write canon material for the official game supplements so I must admit to some confusion on the matter. I don't think we will ever have a correct answer unless one of the GDW staff supplied an answer.
