I think of commodities as things that can be put into buckets, currencies that can be put into pockets, and contracts that can be put into file folders. If you were to memorize a private key and then destroy it, it becomes only a thought which can only be expressed. Therefor Bitcoin is speech.
In theory, a small scale account ledger can be created between 3 people which could be memorized. There could only be three "thought coins" and the 3 people in the system could memorize who owns which coins.
Bitcoin just scales this thought experiment up massively, but it it just requires computers to memorize who owns how many bitcoins.
Edit: How would one argue the bitcoin account ledger is different from a bank account ledger? Is the bank account ledger also speech? I am thinking probably not.