>Let's say person X has lots of bitcoins (potentially mined from the early days, when it was very easy to obtain). They go and sell it for $30,000, in cash. Let's assume X has the intention to declare the gains in their upcoming tax returns.

it's not what's happening here. i haven't seen where that was implied either.

>it seems that in this case they were trying to entice the guy with stolen credit cards. As that didn't work, they tried switching to money laundering.

no, they didn't entice him stolen credit cards, they, undercover, purported the sale of bitcoins/cash to him for the purpose of laundering the proceeds of the crime ( stolen credit card). Therefore, if he engaged in that sale knowingly, he willingly engaged in the laundering of proceeds of crime, i.e: money laundering. What they were weak or unable to demonstrate substantially was that his transactions thus far before the sting, were largely derived in the operations of aiding and abetting the laundering of the proceeds of crime.