You have to take the context of it. Only safe in terms of the fact that Poloniex will use the correct blockchain and the user is safer from making a mistake. At ALL times your coins are safer (in terms of loss&security) by keeping the coins in your wallet and safer in a “paper” wallet. My use of safe was very qualified in the above post, which is why I used quotes around the word.
If the Launcher and/or group that controls the coins is looking for that transaction’s associated trezor signature, then coins/accounts cannot be stolen by a hacked computer. Further, the devices display the amount and to whom you are sending the transaction limiting further a full breach.