The Court of Arbitration for Sport found that City had not breached UEFA's financial fair play regulations between 2012 and 2016 after initially being handed the punishment in February.
"When I read the reasoning by the international sports court CAS why it withdrew this sentence, I have to ask myself what all this is about," Hoeness said to DAZN and Goal.
"Ultimately, UEFA's reasoning was simply weak. In principle, the CAS saw it the same way, but did not provide any solid evidence of Manchester City's behaviour.
"If the judges at CAS have come to the conclusion that UEFA did a poor job, one need not wonder what the result is.
"The verdict was a clear slap in the face for UEFA. We still have to do with Manchester City - and the club can't help it if things are poorly prepared."