Why does Dana say this was "some kind of reverse symbolism" when she is called back to Rufus for the last time on July 4, Independence Day in Kindred?

It is an example of reverse symbolism because Fourth of July is
Independence Day, and Rufus called Dana back to a time when she would be considered a
slave.

Symbolism is when something stands for something else. 
In this case, Independence Day does not stand for freedom, but instead for slavery.  This is why
Dana calls it reverse symbolism.

Dana has gotten used...