There's two viable, existing routes for trains between the Central Sweden and Northern Germany. One's through the Fehmarn Belt rail ferry, and another is the long way through Jutland. Both ways take a long time compared to the distance covered. Stockholm is already ~5 hours by fast train from Copenhagen -- this segment of the journey stays the same. It's after that things get complicated and slow. Operational issues remain with changing voltages and train protection systems (ETCS Level 2 will be built out in a few years), or with the ferry.
A Fehmarn Belt fixed link would be a boon for such a service, like the article says. But it remains to be seen whether that will be built.
[1] https://back-on-track.eu/night-trains-to-europe-new-opportun...