No,
conspiracy liability makes Assange liable for for whatever the charged
conspiracy, which included Manning, did. The "overt acts" are those things the prosecution can prove
Assange himself did. They're the glue that connects Assange to the conspiracy.
"Title 18" is almost the entire federal criminal code. Saying "a specific clause in title 18" is like saying "somewhere, in the entire US federal criminal code, it says...".
As I just said: the murder-for-hire scheme --- which I believe was in fact part of Ulbricht's sentencing --- was an "overt act" in Ulbricht's conspiracy charge.