Is this the case? Can it not give out seed phrase with just a power source?
You can literally generate compatible seed words offline and import them later to your Trezor device.
Internet connection is going to be needed later for sending transaction and for checking your coin balance.
If you want to be fully airgapped and without any connection there are airgapped wallets like Passport, Keystone, Jade, Seedsigner and few others.
Before I had a hardwallet I used to do this (generate seeds offline)
I would install Linux (tails, mint, ubuntu etc) on a flashstick, generate several wallets, write down the seeds offline and use them as needed
But honestly, having a Trezor, I don't know if OP need to be that paranoid
But it's a valid alternative