Well, that's what I'm saying.
BTC has a market value of X, which is the amount of goods that THE MARKET says it can buy. It doesn't matter how you denominate it: USD, GBP, tons of gold, oranges or paper clips.
Say that the USD Govt holds BTC worth 1/10 * X, and wants to borrow 10 * X.
If the market believes the USD Govt can leverage that BTC 100 times, then good for them. They'll be able to borrow. But the VALUE of BTC will continue to be X.
If then the Govt turns around and decides to hyperinflate the USD to repay, it definitely can. Lenders will get rekt, the economy will completely collapse, and probably whoever did the hyperinflating will end up hanging by the neck.
What may happen then is that because the USD stops being a usable unit of account, people decide to simply switch to BTC for that purpose.
But BTC keeps buying X amount of goods. That doesn't change.