This is a reference to the weird situation that Schleswig Holstein was really the only state that allowed gambling until 2021 (there were exceptions). Every (legal) online gambling platform officially required you to be from Schleswig Holstein, otherwise, you were not permitted to join.

Of course, this went exactly as you'd think. However, since 2021, you can get a gambling license in all of Germany.

Fun fact: Since it's illegal to gamble with a non-licensed provider, using international platforms like polymarket can get you in trouble.