The point is that speech from a righteous heart is the best kind, the most effective. By righteous, he means honesty and integrity.
The Roman orator Quintilian was known for something similar about communication: "The good man speaking well." Honest speech from a "good" or honest person is the ultimate goal.
Effective communication always starts within, and flows from the person we are and will become. You want to improve your communication skills? Work on improving the inner you.