I'm not sure who "they" are in this question. I assume Assange knows who his sources are, and as someone who claims to be a journalist and run a journalistic enterprise, he would not accuse people of something he had no evidence of.
At this point, there's plenty of investigations as to whether he was a source, many of which are shown on the relevant portion of his Wikipedia page.[1] Either all the evidence so far is incorrect, or Assange knew he wasn't a source, or Assange didn't know whether he was or was not a source, and in either case, anyone that claims to be a journalist or working towards exposing the truth should not be making statements such as he did, or at least that's the opinion of many people.
1: https://en.wikipedia.org/wiki/Murder_of_Seth_Rich#Debunking