I said secession mitigates tyranny of the majority.
I don't think you understand what mitigate means. It doesn't mean the risk is eliminated. Quite often a mitigation effort will do the opposite (i.e. vaccines mitigate risk of death but some patient has allergic reaction to vaccine and patient dies).
Arguably had black people been able to secede from the union AND the confederacy they might have been better off, but of course, both the union and the confederates would likely have attacked them in that case, because it's not as if white people didn't want to tyrannize blacks even in the north, they just didn't do it with the mechanism of slavery (arguably because slavery was just becoming bad economics in the industrial north and attacking slavery was more of a way to get one up on the south than to help black people that hardly had the rights of whites in the North).
You did say that the secession of the South mitigated tyranny of the majority, when in fact it did the exact opposite.
If you want to say that it usually does this, that would be one thing, but you put it as a universal and chose the worst example for it.