Choosing not to tell a lie can sometimes be a private act. A person might even feel proud about being honest.
However, this command is more specific. It forbids giving false testimony against your neighbor. This refers especially to lying in court or in a public setting where someone’s reputation or life may be at risk.
Refusing to testify falsely is an unselfish act. It shows that you care more about your neighbor’s reputation and well-being than about protecting yourself or gaining an advantage.