You just argued against your own point. It's exactly what I've been saying.
Why should a bitcoin-based island big mac fluctuate when an aud-based australian big mac doesn't?
What I'm saying is simply that the price of things, such as a Big Mac, remains fairly stable over time when the price is in ordinary currencies.
If a Big Mac, or other ordinary goods or service, is priced in bitcoins, the price will change almost on a daily basis.
As an experiment: How many bitcoins does a Big Mac cost today? How many bitcoins did it cost one month ago? One month ago the value of one bitcoin was $1. Today the value of one bitcoin is $7! That's an enormous deflation rate. And it could continue. And no economy can in practice be made isolated from the USD. So what I'm saying is that it would be impractical to have the price of things in the real world in bitcoins. On the Internet it's easier to have prices in bitcoins because the price can be made to change automatically and dynamically minute by minute.