Did you read the whole article? There is no reference to paper currency, the cash strictly refers to coins.

But you're right, it's not exactly clear what he means there:

"When the Roman Empire fell apart, there were no coins in circulation, just gold. But you didn’t pay taxes with gold. What the Frisians did was reinvent cash. Not just coins, but cash—money you carried with you in order to do business."

I'm pretty sure the Frisians weren't the first to mint coins, and if this refers to a new kind of use of coins, it's not explained in the article.