You can absolutely get an Other Than Honorable discharge as a result of Non-Judicial Punishment given by a Commanding Officer. You have the right to demand a courts martial in lieu of NJP but could face much more serious consequences as a result of the courts martial process. If I was involved in seditious activity, I would take the OTH and be on my way.
I think that is what I always understood. You can opt for a court martial if you are threatened with a less than honorable discharge, but it's like plea bargaining, and you don't know what the alternative might bring.