Your link is the DoJ summary of the indictment, if you read the attached actual indictment they reference title 18 section 793 for count 1 then 3-18. Reading the indictment is how I found the relevant law.

Read it again. The core of the indictment is for conspiracy and incitement. If Assange had merely received unsolicited classified information and published it then he wouldn't have been indicted.

It repeatedly says that Assange "knowingly and unlawfully received documents" in charges 5-8. Something it could be argued these people did too.

And trying to argue "the law itself can be used to charge people, but not the subsections that clearly apply in this case despite the lack of case law striking them down" is silly. Those laws are on the books, and being used in an indictment. They definitely could be charged, though probably won't be.