If someone steal your Ledger Nano S, they will need either:
a. The PIN to unlock the wallet.
or
b. The seed, so they can reset the device and restore the wallet with it.
If you have the seed, you don't really need the PIN, as you can: reset the wallet, restore it and setup a new PIN.
Or the 24 word seed is all they need and they can just create another PIN?
That's right.