Holy eff.
The statement Wainwright gave to KSUPD took place after OPPD asked KSUPD on 1/26 to question him and before the story came out about the investigation and Wainwright's involvement and KSU saying that Wainwright had cooperated with authorities. That cooperation is KSUPD's interview of Wainwright.
The statement is not a statement he gave after being arrested. The statement was used in the affidavit that supported the filing of the case on 3/27, for which he was arrested on 3/28.
Perhaps you're unaware that it's common for arrests to happen after a case is filed, under the prosecutor's direction. Arresting prior to there being a case is usually when someone is caught in the act and/or could flee. Alternatively, when there's an investigation to be done, the cops will do that and present their findings to the prosecutor, who'll direct on what else can or should be done in order for the prosecutor to file the case at the ideal time.
It doesn't do any good for cops to arrest people during investigations without knowing if the prosecutor will actually file the case. The prosecutor's case is what defines what a defendant is charged with, not a cop's opinion.
The January cops did not and could not have told KSU charges wouldn't be pursued. KSU took advantage of Wainwright not having been charged yet in January while there were still minutes to be played and while the roster spot couldn't have been filled. In April, with no minutes to be played for months and players available to fill a roster spot, the procedural step of the criminal complaint being filed was when KSU chose to announce its intolerance for the alleged behavior and send Amaad on his way.