Beats me. Its reads like a pretty nonsensical decision.
“Texas had never been outside the Union” is a pretty absurd statement, given that state that Texas was in at the time (occupied and without representation in Congress).
On the other hand, I could just be wrong:
Chase did oppose Davis’s trial, both privately and in his official capacity as Chief Justice, but not because he endorsed Davis’s take on the legality of secession.[15] Privately, he agreed with those in the north who favored leniency for former rebels.[16] Publicly, as one of the presiding judges in Davis’s trial, Chase also favored ending the proceedings. His reason for doing so, however, turned not upon the question of secession, but rather on his interpretation of the Fourteenth Amendment’s third section.[17] This provision, which barred former Confederates like Davis from holding public office (absent approval by two-thirds of Congress), was viewed by Chase as imposing a “punishment” for treason. That being the case, a treason trial of Davis would violate the Fifth Amendment’s prohibition upon Double Jeopardy, since it would seek to punish him again for the same offense.[18] (Similarly, when seeking to have the treason indictment dismissed, Davis’s lawyers didn’t raise the constitutionality of secession as a defense.[19] Rather, they focused on the Fourteenth Amendment,[20] along with general considerations of justice.[21])
What's absurd about it?
On the other hand, I could just be wrong...
A good chance, if that's the best you've got. Chief Justice Chase did not and never ruled that secession was legal. As your article pointed out, Davis's release was because Chase was of the opinion that the 14th Amendment meant that any trial and conviction would violate Davis's 5th Amendment protections.