Well, the tournament only had 2 matches before the assassination attempt ruined everything.
The first match Shiyi (in disguise) defeated Ye Lou. In the second match, Feizhen lost to the Qilin assassin. If you consider the Qilin assassins disqualified, that only leaves Shiyi and Feizhen, and since she was in disguise and nobody knows who she is, Feizhen is the only one who actually won a match.
Normally you wouldn't consider him champion on such a flimsy technicality, but since the Emperor desperately needs a scapegoat Feizhen is the best they got.