How would you know which miner would pick up this transaction and process it?
You privately send it to the colluding miner; you don't publicly broadcast it.
That would be visible to those that save the full protocol state / interactions but probably invisible to almost anyone else.
And you could even broadcast it after you know the miner already solved a valid block with it but before they publish it.
You don't have to propagate the transaction to other miners.
I don't know, but it's an unlikely mistake to make, and they have a lot of know how in the business. I expect them to know something we don't about the protocol, and setup things in a way that they get the money with plausible deniability.
Maybe do not publish the tx and be the only one including it in potential blocks until successful.
How is anything "laundered" in that scenario as opposed to simple sending the money as the transaction content?
It's not. This is like not subtle at all, there's a lot of real time monitoring on fees to decide the optimal one for your transactions. If this was not a mistake and it's really someone trying to hide the 23m transaction, they just didn't know what they were doing. A normal transfer would have been sneakier.
If it were money laundering, they would have not broadcast the transaction.
In this case, the transaction was broadcast to the mempool before it was mined.