Context was the aftermath of the Civil War. It really doesn't settle anything regarding the PC, since in order to get there the IA or Martial Law needs to be cited/implemented.. Nothing new there. Can't go into states without IA with Fed troops for these purposes. Specifically the Army, but the Navy/Corps via other statutory limits. This would end with the use of the NG, if the governors weren't in on it.
That looks simple to me. Maybe because I’m totally ignorant?
KEY EXCEPTIONS TO THE POSSE COMITATUS ACT
“Pursuant to the presidential power to quell domestic violence, federal troops are expressly exempt from the prohibitions of Posse Comitatus Act, and this exemption applies equally to active-duty military and federalized National Guard troops.”