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.
Maybe do not publish the tx and be the only one including it in potential blocks until successful.
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.