"I’m forced to wonder if their theories are simply immune to falsification."
Ouch! A fine summary of the Krugman/Keen debate. I take this to be the central proposition in Keen's paper:
Banks play a crucial role in Minsky’s analysis because they can endogenously expand the money supply in response to entrepreneurial or Ponzi Finance demands for funds. This emphasis upon the crucial role of banks can be traced back to Minsky’s PhD advisor Schumpeter, who argued that investment is not financed by savings, but by the endogenous expansion of the money supply by banks.
In other words, banks don't need deposits in order to lend.
Winners don't have to do strawmanning would be my takeaway here; Krugman did an awful lot of it. Also, I like operational explanations of how banks really work, rather than diagrams.
NOTE Somebody smarter than me is going to have to put this in the MMT context.