If Nadeem Kachi wanted to go after members crhSteetWalker and Omega for defamation, it would be very easy for him to have an attorney get a court order for their sign-on details (email, IP address, etc) in order to determine their identity. Kachi would file a civil action against John Doe1 and John Doe2 known online as crhSteetWalker and Omega respectively and would subpoena TreasureNet for evidence to be used to determine the John Doe identities.
Because defamation is a civil tort rather than a criminal offense, the rules of evidence would allow for a finding of libel purely on circumstantial grounds. For example, if Kachi's attorney could show that there was a beef of any sort between crhSW and/or Omega (motive) and could show that crhSW and/or Omega had the ability (means) and opportunity to put the defaming image and statement on the coins and then distribute the coins, it could be put to a jury to determine the likelihood that those individuals did the defamation and what the damages would be. It would come down to credibility in a court room - not much doubt on who wins that battle.