I've read several documents and declarations from this era, and I might have read that one. Your point naturally being that it states they are seceding to preserve the institution of slavery.
Well funny thing, when the US seceded from England, all those states were slave states too, yet they never get tarnished with assertions of hypocrisy, and nobody questions their right to secede.
Yes, slavery was an evil institution, but yes, people had a right to secede, even if it was for a very bad reason.
It is just like a wife who wants to leave her husband. She might be an evil b*tch who does drugs and sleeps around, but she still has a right to leave if she wants to.
Her evil ways do not abrogate her right to leave.
Similarly, the slaves had a God given right to overthrow their "masters", by force if necessary.
Had I been around at the time, I hope that I would have supported such regime change.