I would argue that buying BTC is in fact a hedge against USD. Many people have used Gold as a hedge in the same way.
Since BTC (and Gold) are priced in USD, you are in effect "taking the other side of the trade". On the one hand,
you hope your USD holds its value. But if it doesn't, your BTC increase.
Gold isn't used as a hedge against USD. You are thinking of rotation. When investors rotate or re-allocate their funds on the belief of rising or falling prices amongst different asset classes. A lot of investors rotate from equities to gold & vice versa.
"Taking other side of trade" is literal. Its being both long & short on a trade simultaneously. Normally your initial position is long term and your hedge is short term.