You have to consider a few things:
1) Sean Miller is risking absolutely nothing by making this public statement. His reputation and career is already ruined if this report is true. And he's not breaking any laws by Denying it outside a courtroom...
2) The University of Arizona is the one in a tough spot. They may not know exactly what is or is not on tape. They have two choices: Fire Miller or keep him. There is tremendous risk in both.
3). The FBI is not going to release or leak that tape as its likely key evidence on their case involving Book Richardson
4). I would hope ESPN has triple sourced this report. If not and they are wrong...hoo boy...