It’s correct what you said.
without the Bridge, you could perhaps add something via the Bluetooth-binding - but I doubt it as the Nuki Smartlock is protected by encryption.
So: please have a look in the Forum for “Nuki” - at present there’s a beta Nuki-binding (which is at least in my environment not stable).
So, basically read the manual for the API (take the Bridge HTTP-API - I don’t link it here as it changes constantly) and add items accordingly. But first you could give the Binding a shot, perhaps it works for you as is.
PS: Don’t use the IFTTT-API, it’s awfully slow.
PPS: I just saw, there’s a new Bluetooth-API, so what I said on Bluetooth-binding could be proven wrong if there’s a Bluetooth API in place, which you can use without a Nuki bridge - BUT Keep in mind: using Bluetooth with the smartlock directly Drains the battery faster!