Unfortunately the speech does nothing to alleviate my concerns. In fact, it is what triggered them in the first place. Antonin merely demonstrates that the PIN code is entered on the computer, claiming it is a safety measure in case the device gets stolen. The scenario I mentioned is not discussed in the presentation.
/.../ and PIN is another protection for case when you lose this device or it's stolen. This gives you some time to reconstruct this wallet in some software and send money out of this wallet into some safe place and do it more quickly than someone who found this device and tried all PIN combinations.
That does not explain why the PIN code is not entered on the Trezor itself, as suggested in my original post.