I have heard this theory a few times. Where does this come from? Why would you publicly purchase one of the most visible and famous football clubs, during the hype after they've won a league title, to... launder money?
When drug dealers pour money into football clubs, like in Colombia, they brought more attention onto themselves and made themselves a bigger target for authorities. There's also the argument that they used their patronage of football clubs not just for laundering purposes, but for soft-power, to engender support from the people in their cities. Similar to what Abramovich did. Redbird isn't exactly engendering a lot of public goodwill with how they are running things, so I find those theories to be odd.
Also, the money behind Redbird is all... straight-laced and conventional. Like... the New York Yankees and pension funds, lol.
If Milan were doing the shit that, oh I dunno... Juve, or Inter, or Napoli are being investigated for, then I'd get the money laundering angle. Your inflating transfer fees, trying to hide your tracks, while getting investigated by authorities from different countries. But Redbird is just being... stingy. Cash has to be moving in order for laundering to work.
I would love for some consequence-free money laundering for some transfers right now, lol.