I like your analysis and think it's spot on. I think the only way op can say it's not 10k is if it was not a "real" coin. For example, if he accepted squidcoin from a 3rd party 2 hours before the founders did a rugpull, that's just bad luck. But if he accepted payment from a fake coin made by the payer with basically 0 volume, he may have a case to say it was fraud.
I found this guidance from the irs which also supports what you've been saying.
A26. If you receive cryptocurrency in a transaction facilitated by a cryptocurrency exchange, the value of the cryptocurrency is the amount that is recorded by the cryptocurrency exchange for that transaction in U.S. dollars. If the transaction is facilitated by a centralized or decentralized cryptocurrency exchange but is not recorded on a distributed ledger or is otherwise an off-chain transaction, then the fair market value is the amount the cryptocurrency was trading for on the exchange at the date and time the transaction would have been recorded on the ledger if it had been an on-chain transaction.