It's amazing what people will dismiss as "unsustainable revenue models." A business that makes money from every single user, and does so in a way that scales exactly linearly with usage? "Unsustainable!"
Now, you seem to have interpreted that statement correctly anyways. So do you have any evidence whatsoever that Square "makes money" (ie. turns a profit) from "every single user"?
As for the source, the slide was on TC and the report is available with a subscription at https://intelligence.businessinsider.com/welcome. I can't verify it, but everything I have seen/know about their revenue model makes it look genuine.