The issue before the court in this case was not secession. The court did not address the constitutionality of secession. The issue at bar was the recompense to White (a popular litigant in this period for some reason </sarcasm>) for the bonds. To determine whether or not the court had jurisdiction to hear the case the court maintained that the acts of the people of the state were invalid. I find it extremely amusing that Justice Grier (of all people) held that Texas had seceded despite his holding in the Prize Cases.
Nolu chan wrote a wonderful expose on the actions of White and the dubious quality of the several cases, especially considering the amount of compensation was less than the costs of the litigation.
Nolu chan got canned a long time ago. You need to check out the WSR threads more often.