It's certainly a bit of judicial activism, but look at it from the point-of-view of the 1869 Supreme Court. The nation had come out of Civil War five years prior, a politically and emotionally contentious "rebuilding" was taking place in the former Confederate states, and here comes a lawsuit based upon a provision of the Constitution that, if upheld, would have crippled the court system even back then due to inflation. The Court's best option was to proactively dismiss that unworkable element of the Constitution that, while important when written, had lost almost all of its relevance, as there was no monarchy ruling in favor of itself. Their other option, of course, would have been to require the jury trial, thus opening the floodgates and bringing an already-overworked court system to its knees. That, in turn, would have forced a Constitutional amendment over something ridiculously trivial, a process which can easily take seven years on a good day, and would realistically have taken much longer because, well, the states had bigger things to worry about.

Typically, I'm against judicial activism. In that case, though, it was the only decision that made sense considering the circumstances surrounding it.

*Edit to add: while it does set kind of a frightening precedent to allow the Court to effectively nullify parts of the Constitution, it's one that the Court has, wisely, never employed.