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.