1. Assange really will choose to say who is or isn't a source (that carries its own risks in so many directions...).
2. Assange actually wants to tell the truth about the source...
3. Assange actually KNOWS the nature of the source? If Bob comes to him and says he's from Brazil (random country selected) and he got the info from Australia (another rando country) ... how sure can Assange be?
I'm not sure I find Assange to be a reliable narrator. The nature of wikileaks has for a while felt like it wants to craft its own narrative rather than just release information. Even as far back as when they were on Facebook (not sure if they still are) any question that seemed to even be distantly critical was deleted. They'd announce a big upcoming leak and nothing would happen ... if you asked anything about it your post was deleted.
I felt like they were playing their own game(s).
But even if that isn't the case, I don't trust that Assange can be as sure as he sounds about where information comes from. If he just up and trusts what his source tells him or just assumes he has the resources to be sure, that's kinda naive...