My mistake but if I'm not mistaken this is not the first time he has made a statement on the topic and they seem to say the same thing..I probably assumed it was an older statement.
Your "square one" reasoning is not as simple as you think. Firstly, there is nothing to suggest that Vialli is telling the truth OR that he was told of all that he was taking. If experts reports, tests, analysts show the opposite as it has..its really not back to square one. More over, its up to the court to determine the probative force of testimony. e.g. Just because Mr. X said he never met Mr. Y it does not mean the court MUST conclude they never met.
In addition, Vialli's testimony MAY actually help the prosecutor. They don't claim that the players were behind the scandal but rather that management in question were evasive with them and supplied the illegal substances. Thus, showing that the players honestly believed they were talking legal substances but reports show the opposite it goes towards the cover-up.