The definition of velocity is not at issue, the relationship between "price" and transaction volume is. If tomorrow the US gov decreed that every $1 of USD will immediately be exchanged for $2 of USDX you wouldn't expect, in theory, for the USDX transaction volume, denominated in USD, to change.
Put another way, I fail to see how the BTC/USD exchange rate is related to BTC transaction volume except for tangential reasons like sentiment.