In this case, the money-laundering statute with which Chastain is being charged -- 18 USC 1956(a)(1)(B)(i) -- does require that the transaction was originally "designed in whole or in part" for a purpose that is itself illegal, namely the wire fraud.
The wire fraud indictment, in turn, seems to hinge on the allegations that Chastain used "false and fraudulent pretenses, representations and promises" (namely, using anonymous accounts to flip the NFTs) and acted "in violation of the duties he owed to OpenSea" (namely, by using OpenSea's confidential data for personal gain, contrary to an agreement he signed).
I’m under the impression that as an employee also you don’t have to sign a specific agreement not to use company insider information for personal gain, but having signed that agreement strengthens the case that the accused knew what he was doing was illegal.
The wire fraud indictment, in turn, seems to hinge on the allegations that Chastain used "false and fraudulent pretenses, representations and promises" (namely, using anonymous accounts to flip the NFTs) and acted "in violation of the duties he owed to OpenSea" (namely, by using OpenSea's confidential data for personal gain, contrary to an agreement he signed).