Seems odd that there is even an argument. Patrick Harnett is supposed to have paid back some Bitcoins and has not. I am sure there are reasons for this but that still does mean he is in default. Paying back little bits of what he owes doesn't change that.
Did you read the thread you are responding to? The agreement by which he was supposed to have paid back some Bitcoins is not enforceable as agreed due to common mistake.
Or, do Bitcoins operate on a different set of logic?
Of course they do. Otherwise the agreement would be void because of the usurious interest rate.
Why does a "common mistake" matter? Anyone lending/giving/investing Bitcoin probably believes that the other side can live up to their end of the agreement but when they don't it's the fault of the person receiving the Bitcoin who is then unable to pay back what they owe.