I don't buy that for a second. "Blacks were better off slaves in America than free in Africa" is an all too common Republican trope thrown around in the face of... well, blacks. And a book on display just so happens to randomly end up on that exact page? Not likely.
Ultimately it sounds like a justification or an excuse or a diminishing of the horrors of slavery. Now, granted, Rome and much (most... all?) of humanity were comprised of slave societies at one point or another. But the fundamental principles of modern Western Civilization is to abolish slavery, and to enshrine a set of basic human rights for all. For all to be created, and treated, equally. Those principles are the virtues of our civilization. The meaning of that quote spits in the face of all we hold dear.
And it's not just some random phrase, it's a calculated one that is not uncommon and not without predetermined use. It stands as a more effective means to erode or... to white wash the stains of our past. Now maybe this specific display of it is not premeditated, but that is a difficult conclusion to reach given the circumstances as I know them. The burden of proof should be on the Congressman to explain why it got there, and what it means to him. What else, in particular, turned him to that page? Anything less than both a full apology and explanation would simply mean that he is... outed. Shamed.
OTOH, if he has a history of helping blacks and other minorities than perhaps we can reconsider the odds and the... burden of proof. But he is a member of the GOP and so the expectations there are... low. How would he vote on such issues as criminal justice reform, voters rights, etc? There are ways to figure out and guess his penchant for being a good person, or not, based on an existing record.