This is indeed a motorhome/autocamper stopover.

The "no camping" sign only means that you cannot use tents, bbqs, etc. You can park any car there, including motorhomes.

At the Nysted harbour payment terminal you can buy tickets for autocampers for 145 DKK per day.

https://www.havneguide.dk/havn/nysted-havn

This has been discussed in the local newspaper.

https://folketidende.dk/guldborgsund/fortsat-plads-til-autocampere-paa-havnefront

Note the area E that is designated for campers.