Well, yes. For the Etihad sponsorship, where on earth would they get the evidence that Mansour paid most of the money to Etihad?
They have some emails that say an "HH" arranged for the monies, some bank statements that follow the numbers in the emails (8 from Etihad and 52 from central funds) but emails don't prove what happened even if they mean what the PL say they may mean. They have no idea at all where Etihad got the money. There will be nothing at the club that shows that, nor should there be, and they don't have the power to demand information from Mansour or Etihad. I think we can discount a whistle-blower. So the only explanation I can provide is that they decided to refer the allegations to close the investigation by "forcing" the club to provide the withheld evidence to the IC. The alternative would have been an embarrassing climbdown for the PL.
The outcome, though, will be an embarrassing "defeat" for the PL which is why the club have been so clever.