What? Which ledger are we talking about? Ledger that makes Nano S and so on? From where did you read that they know the key phrase generated by its users? If that's true don't you think no one would buy their wallets?

You don't have to be online to create a new wallet (where did you learn this?). You don't need to be online to sign a transaction either. Maybe you can read this for additional info https://bitcointalk.org/index.php?topic=5190202.0.