I'd love to buy a Tesla with my Bitcoin, but it would make taxation so much harder, so I'll stay with using an exchange :(
In the US, buying something with bitcoin is the same as selling your bitcoin for cash.
I think that's his point. when he deals with an exchange, they deal with all the accounting, i.e. reporting how much he made or lost on the transaction. if he pays for it at Tesla, he has to handle all that accounting himself.
They don't deal with it, but at least you can export the data in a format that is clear to accountants, and tax authorities can check the bank accounts. I'm not in US, and I wouldn't want to explain the details of Bitcoin transactions to tax authorities that don't know it well.