Only Assange knows where he got the data. Why is it so easy for you to trust entities that were not involved in the leak so easily, yet disregard Assange's own agency over knowing/understanding where he got the data?
Is it because you maybe just don't like Assange as a person?
I mean, not vetting documents is one thing. But, actively engaging in election interference all over the world is another thing. Assange did the former - your beloved intelligence agency, are currently doing the latter. Its reasonable to assume that US Intelligence agencies are lying, utterly, to give themselves yet more budget for nefarious deeds - i.e. the "Ma' Russkies" narrative, which serves only to foment more hatred...
I think Assange isn't particularly careful or picky about what he releases and by extension particular inclined to thoroughly examine who's giving him that info. Additionally from all appearances he scheduled and coordinated (at the very least indicated he had something) to affect the US election.
As for my feelings about the various TLAs in the US they're mixed but I definitely think they're more likely to be able to tell who was actually involved than Assange.
[0] Wikileaks isn't a huge organization with massive investigatory prowess and resources, it wouldn't be terribly hard to believably fake non-association for a nation state.
The same agencies that said, with certainty, that there were WMD's in Iraq .. and then, after a few million innocent people have been murdered, can't find them?
I think Americans put too much faith in their TLA's. These organisations are as corruptible as any other, and - as we have learned - could just as easily be controlled by Russia as anyone else.
https://www.washingtonpost.com/news/post-politics/wp/2017/03...
But I can't find anything about physical destruction, CrowdStrike claims to only have ever had images of the systems.
https://www.crowdstrike.com/blog/bears-midst-intrusion-democ...
In 2018, the DNC said βThe FBI was given images of servers, forensic copies, as well as a host of other forensic information we collected from our systems.β
https://www.thedailybeast.com/trumps-missing-dnc-server-is-n...