I also love that this ruling explicitly said that the act of levying war against the United States is treason.
"When war is levied, all those who perform any part, however minute or however remote from the scene of action, and who are actually leagued in the general conspiracy, are traitors.
Any assemblage of men for the purpose of revolutionizing by force the government established by the United States in any of its territories, although as a step to or the means of executing some greater projects, amounts to levying war. The traveling of individuals to the place of rendezvous is not sufficient, but the meeting of particular bodies of men and their marching from places of partial to a place of general rendezvous is such an assemblage as constitutes a levying of war."
This means that anyone who aided or abetted actions detrimental to the war effort could have been charged for treason. This is a VERY broad brush, more so than Lincoln actually used.
And of course Merryman didn't actually enjoin Lincoln to release Merryman. Only that he really shouldn't have done that. SCOTUS has never taken it up, and Congress itself provided political cover for Lincoln later on.