This is true. A couple of years ago Tether (along with its sister company Bitfinex, a crypto currency exchange) settled allegations by the New York state in the U.S. by paying a fine (in the double-digit millions), admitting that claims that Tether was backed by fiat at all times was "a lie".
A major issue with this coin is that it is not subject to regulation by any authority (it's owned by iFinex based in the British Virgin Islands), so they may claim whatever they want.
I don't know what exactly made them choose Tether, but one reason might indeed be that they don't have much choice (alternative crypto coins are arguably far too volatile to serve as a means of payment for companies with higher bills). Maybe because the company has an office in Hong Kong as far ad I know (at least they had one not long ago). Maybe also because there is a higher volume, maybe because there is also a Tether variant pegged to the Chinese yuan (it hss the same shortcomings as the USDT, but a much lower volume). I don't know.
But let's not forget that it can be tracked as it's on a blockchain. If they seek to circumvent sanctions and hide their money trail, that's not a good idea.
In a nutshell: anyone who says that the sanctions don't work should read stories like that and they might change their mind.