Believe it or not…there are churches that don’t bear false witness against people who have moved their membership to a different congregation/denomination, or against people who resigned their membership for whatever reason. They just don’t.
But you are correct about the bearing of false witness. This happens a lot in Adventism in several different situations (people who leave, or people who have a critique, or disagree with their doctrines). It’s strange that they don’t get that they are breaking this commandment, especially because they are so dogmatic about keeping the commandments, and claim that they are the only group that keeps them all. But, they don’t.