The tax calculator excludes all principle. It makes no distinction. Loan principle includes all sorts of things, just not housing. Cars, clothes, meals, housing, etc. During the transition, all goods bought under the old system did not have sales tax (because there was no fairtax yet), but as time goes by more and more of the principle will be goods with the fairtax. It is dishonest analysis to include principle as things that aren't taxed under the fairtax.
The calculator also has a mathematical error. You receive the "prebate" but they assume you never spend it (you pay FairTax on it when you spend it thus reducing it's real value).