By your logic if that were the case, Tether would need to be destroyed everytime the crypto-markets rise...
Plus, Tether's offical website and documentation state that's not how Tether is intended, they claim it is always 1:1 backed by a real USD reserve in a bank account (which they tell people they will audit, but the auditor backed out after giving a statement that said Bitfinex would not provide the proper documentation and access, later erasing any mention of having been associated with Bitfinex.)
Plus, Tether's offical website and documentation state that's not how Tether is intended, they claim it is always 1:1 backed by a real USD reserve in a bank account (which they tell people they will audit, but the auditor backed out after giving a statement that said Bitfinex would not provide the proper documentation and access, later erasing any mention of having been associated with Bitfinex.)