They seem to have missed one key detail in the title: this is the last European passenger train that travels by sea - or rather, the last ferry crossing that carries passenger carriages (I doubt that they carry the engine across by ferry?). There are (AFAIK) several other ferry lines that carry freight carriages, among them the Rostock-Trelleborg line served by https://en.wikipedia.org/wiki/MS_Sk%C3%A5ne
> I doubt that they carry the engine across by ferry?
I don't know about this specific instance, but the Germany-Denmark train-on-a-ferry carried the whole Diesel-powered train, engine and all. It drove onto the ferry on its own, and left it on its own as well.
It's not longer than the ferry?
No - it's a rather short train, only a few carriages, and the engine is mostly integrated into the carriages I believe.
They decouple/shunt the carriages so they're alongside each other rather than having to fit them all in end to end.
This is not true, on the route Denmark/Germany there was no decoupling involved - the train drives onto the ferry, and drives off the ferry afterwards.