I agree that his use of "lawless" is odd, especially because he very quickly lays out laws and even explains that some interest is necessary. (the part of interest that pays the working entrepreneur the value of their management labor, the part if interest that pays the wages of the workers maintaining the money infrastructure/banks etc.)

He mentions at one point the bookkeepers in Indian communal villages, and how their substance is assured by surplus labor volunteered by the village. You could call their subsistence costs a form of interest on the total productive output of the village. I think the only think that makes interest "lawless" is situations where the lending capitalist decides to raise their interest rates to increase their personal standard of living. They can do this without having to balance out another commodity (labor price in the industrial capitalists books).

The fact that they limit themselves is some form of class collaboration and knowledge that exceeding a certain amount of interest would deprive them of the production needed to reproduce themselves.

That being said, I definitely find Marx's explanations on the subject lacking, even if all the examples he gives work correctly.