Please ELI5. What I have learned is that the added passphrase, or the "25th seed word", is a feature all BIP-39 compliant wallets have that generates a new set of keys. It's doesn't matter if the user owns/uses a Trezor, a Ledger, or any kind of hardware or software wallet. If the user enters his 24 seeds + his passphrase, the output will always be his/her "hidden wallet". Are you saying it's not?
The "25th word" is a misnomer like the "13th word" and it usually used to mean the password that is used alongside a BIP39 mnemonic to derive the master public key.
By using a PIN to encrypt the BIP39 password, it reduces the security of said password to the strength of the PIN, especially since most PINs are short sequences of numbers.
BTW: This whole "Open Source Roadmap" is missing one important entry: To open-source the entire Ledger firmware!