The prosecution memo does not rebut this argument, it rebuts instead the clearly different but related argument that the murder-for-hire scheme was uncharged conduct which could therefore not be considered in sentencing. It was -- as they correctly point out -- charged, as it was one of the overt acts specifically laid out in the Count One narcotics trafficking conspiracy charge.
It is nevertheless inaccurate to say he was convicted of hiring a hitman, since a conspiracy conviction requires (as far as overt acts go) only a finding that the defendant committed at least one overt act in furtherance of the conspiracy, there were several overt acts charged in that count, and the verdict form did not direct the jury to return separate findings of fact on each charged overt act.
The argument I'm challenging is the notion that the factual claim of Ulbricht's attempt to hire a hitman wasn't subjected to scrutiny during the trial. It was a specifically introduced factual claim, which Ulbricht's counsel was required to rebut.