You can downgrade it without knowing the PIN? That would mean any security mechanisms implemented through firmware upgrades would be pointless, since an attacker could just downgrade to an older version and exploit the vulnerabilities that were fixed through upgrades; I can't imagine that's possible.
Good point! I forgot about the PIN. I don't own a Trezor device but when you install new firmware updates on Ledger, it does ask you to enter your current PIN. I am sure it wouldn't work on Trezor either. To get the firmware installation files, you would have to connect to the Suite or the wallet.trezor interface. With an unlocked device, that would surely not work.
If you can downgrade, you will be able to attack like in the video. The update doesn't 'remove' the data permanently from insecure storage or something like that; v1.6.0 copies the secure data into RAM at boot, it will do that no matter if it was updated and downgraded again.
As o_e_l_e_o pointed out, it's not possible to downgrade the firmware if you have anything newer than 1.6.1.