Tether's claim is that verified users can redeem USDT for USD 1:1 at https://tether.to/. Exchanges like Kraken are secondary markets and Tether does not claim anything about them.
"Tether reserves the right to delay the redemption or withdrawal of Tether Tokens if such delay is necessitated by the illiquidity or unavailability or loss of any Reserves held by Tether to back the Tether Tokens, and Tether reserves the right to redeem Tether Tokens by in-kind redemptions of securities and other assets held in the Reserves. Tether makes no representations or warranties about whether Tether Tokens that may be traded on the Site may be traded on the Site at any point in the future, if at all."
Oh, and don't forget the fees:
"Fee per fiat withdrawal: The greater of $1,000 or 0.1%"
