That’s absolutely not true. Just read the 🦆ing law. The fines can be found in BKatV, Anlage (zu §1 Absatz 1).

Nr. 100 says “Vorgeschriebenen Sicherheitsgurt während der Fahrt nicht angelegt; 30€” which clearly states that YOU get fined for not wearing a seatbelt.

Nowhere does it say anything about transporting someone else who isn’t wearing a seatbelt, except for children who are explicitly mentioned in Nr. 98 and 99: “Ein Kind mitgenommen ohne für die vorschriftsmäßige Sicherung zu sorgen”.

Even if the clear language of Nr. 100 would have left any doubts, the fact that the numbers talking about children explicitly mentions transporting someone else shows that that’s not the case for grownups.