I think it's even later than that. The events he is talking about occurred in September 1861, after the Confederacy had started the war. So in effect the disloyal members of the Maryland legislature were advocating a vote to join in the war against the U.S. Treason by any definition of the word, much less the definition outlined in the Constitution. So what should the U.S. have done? As it was the treasonous legislators got off easier than they deserved.
I believe there multiple arrests, including of city officials, that preceded the arrest of legislators.