When you define a large mass of people as property and deny them the franchise, that's not oppression through democracy, because its not democracy at all.
In any event it's not necessary to argue this point because there are no shortage of further examples which include authoritarianism exerted over non-disenfranchised demographics. Here are some further examples of authoritarianism within the USA:
* Japanese internment of 1940s
* Every historic invocation of martial law (more than a dozen)
* Every historic invocation of conscription
* Historic oppression of women, which continued well after the suffrage movement in a variety of ways
There are of course countless other examples throughout the history of democracies. Even a cursory reading can uncover countless examples, from contemporary democracies all the way back to its origins in ancient Greece. It is not uncommon for a majority rule system to impose authoritarian controls -- in fact, it is the norm. This is why we have concepts such as constitutional rights, which are specifically designed to balance against the authoritarian tendencies of majority rule.
Its obviously aristocratic republican with a quite open hostility to democracy at the beginning of that period, evolving over time in a rather unequal way in different parts of the union toward democracy (in both ideals and substance, though not in lockstep between the two.)
There is a well known phrase, the "tyranny of the majority." It is in fact extremely common for a majority to oppress a minority within a democratic system -- often with severely authoritarian methods.