That’s correct Matty.
Once you initialize MetaMask wallet, there is a default account created (the red one in the picture). If you wish to use it, then you need to keep your MetaMask seed phrase to recover and access this account whenever needed. This account is not a part of Trezor wallet.
On the contrary, there are Trezor accounts (the yellow and the blue one in the picture).These are Trezor generated accounts and can be recovered (and you can spend from them) only by using Trezor recovery seed.
So you can’t just connect your Trezor to existing MM account. You have to transfer the funds from this default account to account linked to your Trezor (as it’s addresses are derived from Trezor recovery seed, not the Metamask one..)