I can't read Mr. Benson's mind but I would assume that he considered it irrelevent. Mr. Benson was concerned with what was the case in 1860 and up to the time of Davis' release in 1867. The 1869 decision had no relevence to that. The 1869 opinion on secession merely provided a convenient way for the Chase court to protect a favoured moneyed interest. It has no relevence to the larger question of the right of secession.
You may assume all you want, but the fact is that the Supreme Court did determine that secession as practiced by the southern states was illegal, and it managed to do that while not trying and convicting Davis of anything. And since you're assuming for him then how did he come to the conclusion that the legality or illegality of secession was dependent on Jefferson Davis being tried in the first place?