Completely false.
Tether basically runs on the OMNI network and uses the bitcoin blockchain. Basically its validated and secured by the hashpower of all the BTC miners which currently right now is very large and highly protected against 51% attacks.
OMNI network is like a side chain or so. Basically most BTC explorers don't recognise tether transactions because it says error on the inputs but the USDT tether blockchain can decode them properly.
Only issue with this is when the fees goes up, its very expensive to move tether around because the transactions are slightly larger.
Thanks