Nah, it's even simpler than Mises's regression theorem.
The definition of money is "the most sale-able good". Different objects have been the "most sale-able good" at different times and in different areas of the globe, but money has always been the "most sale-able good" under the specific circumstances at the time.
Remember, a "defintion" can be used in place of the word, itself. For example:
"I wish I had more money" = "I wish I had more of the most sale-able good"
This is what he means by "I don't think it fits the DEFINITION of money".
"I do not add 'within the limits of the law,' because law is often but the tyrant's will, and always so when it violates the right of an individual."