The notion that the Iron Bank invests in the slave trade was just assumed by Cersei and neither confirmed or denied by the representative. The silence on the issue could either be acknowledgement that they do invest in slavery OR that they don't.

Cersei may have just shown her hand in insinuating that after she defeats Dany, the slave trade would be allowed to start back up. It's a callback to when Tywin told her that she wasn't as smart as she thinks she is. She thinks she has all the answers and speaks while the Iron Bank rep speaks. Like she knows what he's thinking. She may have just outsmarted herself.