The men do not consider Isabel a person capable of listening and understanding their conversation. They believe this about Isabel because she is a black slave.
Isabel has always known that white slave owners think less of her, but in chapter six Curzon flat out tells Isabel that the Locktons do not consider Isabel a human being. Curzon asks Isabel to spy on the Locktons, but Isabel thinks that she won't hear anything worthwhile.
"They won't say anything in front of me."
"You are a small black girl, Country," he said bitterly. "You are a slave, not a person. They'll say things in front of you they won't say in front of the white servants. 'Cause you don't count to them. It happens all the time to me."
There was truth in his words, hard truth, a hammer striking stone.
It turns out that Curzon is correct because Isabel overhears many important pieces of information while she is the same room as Lockton and his conspirators. She overhears the plot to bribe members of the Patriot army, and she overhears the plot to assassinate General George Washington. The entire assassination plot was discussed while Isabel was in the same room as the men! They don't consider her a person at all. They act like she is the house dog or something like that. Isabel is present, but ultimately doesn't matter.