  Well he believed does not sound very academic but in an attempt to not show bias language is used that neither acts as if it was absolutely true or absolutely false, therefore wording it in a way that shows this might have led to the he believed, I still think it should be more along the lines of, he is said to have witnessed etc. As in how a documentary might say, the lion is taking a nap, we could talk about it without making about his personal belief etc. which I think could be contested, I believe claim is another word used, although not sure on what consensus there is around this.