It is correct, , that it applies to broker contracts from 23.12.2020 onwards. For example, it can be found in the Federal Law Gazette 2020, Part 1, No. 28, §53 transitional provision to the law on broker fees in the mediation of purchase contracts for apartments and single-family houses.
What might be interesting for you is the passage that during the transition period the buyer bears the commission alone only if he agreed to it before it came into effect. But I assume the broker has that in his documents; otherwise, he would be a serious dilettante.
(P.S. I am not a broker, but I can read legal texts)
In the case of the acquaintance, the broker simply did not feel like looking for a buyer any longer. Because for 6% of maybe 10,000€ more (600€), 10 more appointments are not worth it for the broker...
probably clicked send faster ;-)