Three main points:
1) Who cares what their motivation was? O.J. Simpson demanded trial by jury because he wanted to get away with murder; does that mean he should have been denied that right? A right is a right, and rarely is a right invoked by someone in the majority.
2) Yes, the seven Deep South states seceded over slavery. The other four seceding states, however, seceded only after seeing Lincoln’s initial unconstitutional acts — not only attacking the Deep South states (which other southerners agreed had a right to secede), but also raising an army without convening Congress, suspending habeas corpus in Maryland, and otherwise launching his career of unconstitutional acts in opposition to secession. Virginia’s secession convention voted before Lincoln’s inauguration NOT to secede merely in defense of slavery, but reconvened after he’d been in office for a while and seceded because of what he had done. I’m guessing Austin doesn’t know this.
3) Does Austin support the American Revolution? As Holton showed in FORCED FOUNDERS, many people in Virginia supported secession from the British Empire in order to protect slavery. If Austin is consistent, he must decry the American Revolution, at least in Virginia — home of Washington, Jefferson, et al.