Yeah, that's what I understand. Although I know they will deploy other specialties and put them into these roles (ie - the Army will deploy Ob/Gyn as General Surgeons).
I can't imagine what the medicine subspecialties would be able to do in these circumstances. The care, while intense, is necessarily at a lower level due to inadequate facilities and limited followup. You don't need a cardiologist, because they're not going to be doing caths, and you don't need Cardiology to prescribe HCTZ. No point taking critical care if you don't have ventillators and pressors, or an ICU. No reason for ID, since basic antibiotics will be in short supply, to say nothing on linezolid or meropenem.
You need surgery to correct basic things (sOrtho for DDH, Gen Surg for hernias and tumors, trauma surg for bleeding). You need anesthesia to run the cases. And you need primary care to do basic medicine. Everything beyond that is pointless. War zones and extreme circumstances (I would imagine) have a strong triage component, and lot of people die because they lack basic care, not subspecialized care.