"If nobody recognizes a defaulted debt on their balance sheet, does it exist?"
Suppose, for the sake of argument, that there is a world in which banks are allowed by their regulators to pretend their default losses simply do not exist. And, even more outlandishly, some of these banks are allowed to sell heavily damaged loans to their central bank at nearly their full original price.
What does "deflation" mean in such a world? Not much, as it turns out. At least from a monetary perspective, because money is not being destroyed at nearly the rate that would be expected or predicted by the size and rate of the defaults.
This is the world in which we currently live. Trillions in probable and provable losses quietly exist, out of sight, on the balance sheets of the Federal Reserve and other financial institutions. If they ever come out of hiding and onto the books, I think the deflationists will be proven correct beyond all doubt.