A common misconception regarding how Wikipedia works. Wikipedia works by writing what reliable sources has to say about an issue.
In this case, its a newspaper article that are citing Assange, which is why the sentence beginning with "Julian Assange said". Changing it to "Julian Assange allegedly claim" would make the article worse, as the implied disbelief of using the word "allegedly" is not supported by the source.
Having to base edits around sources is hard, but I think a crucial aspect of any encyclopedia. The alternative is just so much worse, don't you agree?
Unless you think "said" implies truth, there is no clear improvement between "Julian said" and "Julian claimed". "Julian said" is not implying that what he was saying is beyond reproach.
To see why it's not worded that way in the Wikipedia article, imagine each sentence in the article implicitly preceded by "According to source x,..." -- source x of course being the relevant citation. If the cited source states that he "said", then using "claims" or "allegedly" does not accurately reflect the degree of certainty expressed in the cited source. Veracity of that source notwithstanding.
It's fairly common to word things that way in Wikipedia articles, though of course not for every citation. It's a judgment call whether it should be or not, since adding a qualifier in-text more strongly contextualizes the claim as being from the source, rather than in "Wikipedia's voice".
For example, I recently wrote an article about an ancient Roman politician. A number of sentences are in the form "[Scholar A] argues that the person did X [cite]" rather than "The person did X [cite]". I'll use the second form for uncontroversial facts, but the first form when it's more like a proposal from a particular scholar which isn't necessarily an accepted consensus viewpoint.
