Even a racist could make that claim. They'd never harbor a slave, so the claim is true, omission of how they'd kill the runaway is incidental to the truth value of the statement.
That's a problem of "truth." Lying by omission is deceptive, and makes you a sophist. You have to guess their lie, but sometimes the lie isn't obvious and you'd need to interrogate them a long time until they lied. If they're lying by omission, what makes you so sure they'll come clean once confronted?
But how do we find sophists without resorting to implementing Thinkpol officers?
As a general rule, lying is wrong when it is used to advance fraud or to aid in an act of aggression against another person etc. Which is why I dislike politicians / coercion based government, as any concept which denies it's inherent coercion is fraudulent from the beginning.
Except deceiving a person who is in the act of using offensive force (slave catcher, drug cop etc.) or is already committing fraud or an act of initiatory violence isn't wrong. It is proper and necessary, since it is repelling the bad. Defensive force to repel "bad" (offensive force) is a natural and rightful thing to do.
We find sophists by exposing them when we know they are lying and communicating their actions to others and by shunning them until they either change their ways or suffer the consequences of their actions. Sometimes, if necessary, we help them suffer the consequences.
"be excellent to each other" - Bill and Ted