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...
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.