You can compute the market price, either by taking the USD/USDT pairing on Kraken directly or by taking the ratio of BTC/USDT on Bitfinex against BTC/USD on Coinbase. They usually give the same answer modulo exchange fees (i.e. within about half a percent or so). When there was a run on Tether in May and it dropped to about $0.93, there was roughly a 7% price premium (several hundred dollars) on Tether-based exchanges like Binance and Bitfinex.
The confusion is because in theory, the market price should be tracking Tether reserves. Bitfinex admitted in a court filing that only 73% of outstanding Tether was backed by dollars. In an efficient market, that should imply that the price of Tether would fall to $0.73. Instead, it went back up to about $1. The implication is that traders either figured Tether would somehow be able to make up the shortfall, or that it just didn't matter.
Interestingly, people who shorted Tether expecting the market to be efficient got rekt, and the people who were actually correct (so far, at least) were the ones who were irrational. I wonder what this says about rationality and efficient markets today.