"Download the latest version of MetaMask for Chrome or Firefox. If you have not used MetaMask before, you will be required to set up a wallet and generate a recovery seed. This recovery seed is separate and is not part of your Trezor and is not necessary if you only intend to use MetaMask with your Trezor."
they say "is not necessary"...
The seedphrase is not necessary to link your MetaMask with your Trezor. That's correct.
If you follow the above instructions I gave you, you should only need to enter your PIN, and authorize the action from the device itself, as you would normally do to authorize a transaction you're sending. You won't need your seedphrase at all.