No it doesn't mean "nothing"........unless you've seceded!
This is not an example of an ex post facto law. It is the court ruling on the Constitutionality of a piece of legislation passed by the state of Texas. They ruled that act invalid.
Point is, the decision was handed down in 1869, Helloooo! So it's not a valid reference when discussing the secession debates, any more than it is valid to teach a course on John Marshall, but then include scholiae all over the place on what Thurgood Marshall would have said about that, or what William O. Douglas invented later.
No more valid, either, than insinuating that the executive orders of Andrew Jackson and James Knox Polk were invalid because they owned slaves, in contravention of the XIIIth Amendment.
And when would you have liked the decision handed down? Before it happened? The Supreme Court rules on cases as they come before them regardless of how long it takes. The ruling in Scott v. Sanford, the Dred Scott decision, was issued in 1857, ruling on something that had happened 11 years before. That's the way it works. The court can't issue a decision on something unless it is brought to them.