Why should that be true?

The point is that as long as the UoA is USD, all prices are fiat denominated in the eend

Telling yourself, "this is BTC-denominated" just glosses over the fact that it's actually BTC-price denominated. Which of course is a USD price.

What we're after is a stable UOA that actually can be used to measure. Until we have that, no prices actually accurately measure the value of a commodity

only in an arbitrary fiat way.