Yes you and others have pointed out this very compelling argument.
It certainly indicates that someone in the right hand seat did it deliberately. The captain believed the FO was occupying that seat and alone on the FD. For an investigator the evidence will be regarded as overwhelming, but not yet totally conclusive for the purposes of an inquiry. Are we certain he was alone on the flight deck - probably - what we have heard so far certainly suggests he was - but I haven't heard the tape myself and therefore it is all third-hand at present.
I assume that even without the FDR there is plenty of evidence from the tape both published and not yet in the public domain which is consistent with that theory.
I also assume that they may have evidence from studying the guy's background and searching his house.
The way to finally get to the bottom of this is not just to establish what happened but why.