Even if the bulk of the key-stretching work has been outsourced to another device, and that device is compromised, the passphrase is now protected by minimum 8192 rounds of salted PBKDF2-HMAC-SHA512. 

The idea is that more powerful devices (mobile phones, laptops, etc.) can do all the key-stretching on their own, whereas weaker devices with access to another device with more computing power (like Trezors) do a fair amount of key-stretching on their own, but can safely export the rest of the key-stretching to the other device.

Will